The issue, as it seems to me, is that almost every text you read on Buddhism does not attempt to do the actual work of translation. The first transmission of Buddhism to the west reified a bunch of translations of terms, such as concentration, equanimity, tranquility, mindfulness, suffering, etc. and works since then have mostly stuck to rearranging these words in different combinations and referencing the same metaphors that have been in use since the time of the Buddha. If these authors had true discernment they would realize that the umpteenth text on 'establishing the noble bases of tranquility secluded from sensuous ignorance' or what-have-you aren't helping anyone who didn't already get the message.
At this point I want to say that I think this approach is 'working' for the fraction of the population it is going to work for. If we want to make the practical fruits of Buddhist practice dramatically more accessible to a broader range of humanity we need people to do the hard work of translation to put the Buddha's teachings in forms that will be accessible to various groups of people.
The hard work of translation is to attempt to use language to point your mind at the same distinctions that the original author was trying to point to. Attempts to do this will inevitably fail in lots of ways, but can hopefully communicate enough of the core message that people can piece together the essential causal relations after which, having had direct experience as a result of skillful practice, they can help to improve the translations further.
So, putting my money where my mouth is, I want to try to produce a translation of what I see as the core causal loop that causes progress on the Buddha's path. I'm attempting this because I believe the core causal loop is actually quite small. The Buddha had a tougher task because he had to explain causation, locus of control, and other critical concepts to farmers from scratch.
To begin with, you may think that the purpose of meditation is to eliminate thoughts. But read the Pali Canon and you find a text rife with concepts, schemas, diagnostic methods for various classifications of mental activity, meditation taxonomies, sensory taxonomies, feedback loops etc. Pretending you're already enlightened and that there isn't hard work to do is something the new agers have borrowed from some shitty spiritual schools of various flavors. I refer to people preaching such messages as mindlessness teachers.
To be clear, a decrease in discursive thought, and especially unpleasant mental contents that don't seem to serve any purpose, are one of many pleasant effects of proper practice, but don't really need to be focused on. It is a benefit that arrives in stages on its own.
So, what is the core loop?
It's basically cognitive behavioral therapy, supercharged with a mental state more intense than most pharmaceuticals.
There are two categories of practice, one for cultivating the useful mental state, the other uses that mental state to investigate the causal linkages between various parts of your perception (physical sensations, emotional tones, and mental reactions) which leads to clearing out of old linkages that weren't constructed well.
You have physical sensations in the course of life. Your nervous system reacts to these sensations with high or low valence (positive, negative, neutral) and arousal (sympathetic and parasympathetic nervous system activation), your mind reacts to these now-emotion-laden sensations with activity (mental image, mental talk) out of which you then build stories to make sense of your situation.
The key insight that drives everything is the knowledge (and later, direct experience) that this system isn't wired up efficiently. Importantly: I don't mean this in a normative way. Like you should wire it the way I say just because, but in the 'this type of circuit only needs 20 nand gates, why are there 60 and why is it shunting excess voltage into the anger circuits over there that have nothing to do with this computation?' way. Regardless of possible arguments over an ultimately 'correct' way to wire everything, there are very low hanging fruit in terms of improvements that will help you effectively pursue *any* other goal you set your mind to.
Funny aside, emotional 'resistance' might be well named, it might be literal electrical resistance in the CNSs wiring as a result of this spaghetti logic.
So back to these stories and story building blocks that are the outputs of this system. You generated a bunch of the primitive building blocks when you were very young and throwing everything together on an as needed basis with no instructions. You both have a back log of such stories and story building-blocks and are generating new ones all the time. Practice improves each of these situations. It improves the backlog by going through and reprocessing stories that aren't actually reality aligned when examined. Again, not pointing to edge cases here but things in the 'your partner humming the spongebob theme shouldn't make you furious because of something that happened when you were 12' class. You can clean up all the obvious stuff and then let your future self (who now has more resources) think about how to wisely deal with the fuzzy edge cases. It improves the new stories coming in (partially by learning as it processes the back log) by building far fewer incoherent stories out of pieces that don't fit together, and building less of the shittier building blocks in the first place.
I'll go ahead and name these things now to connect them up for people who have some knowledge of existing translations.
Concentration meditation gives rise to a mental state where the mind is very calm and inclined to neutrality. Of the same sort you'd want in a good judge.
Insight meditation makes one aware of the causal links in the perceptual system between physical sensations, feelings, and mental reactions.
Sankharas are the stories and story pieces that get reexamined and refactored as a result.
So what is the core loop of meditation practice?
Concentration puts you in the ideal state for insight.
Insight stirs up Sankaras.
Examining Sankharas riles up the mind, eventually leading to a desire to do some more concentration in order to calm down and keep making progress.
Clearing Sankharas cause concentration to go much better. And onward.
Why is concentration ideal to prepare you for insight practice?
Insight requires a high degree of temporal and spatial resolution in order to see the finer linkages between mental activities that normally flow past you without you noticing. Concentration meditation improves that resolution.
Second, to examine the Sankharas is to, to some extent, reactivate the sensations, feelings, and mental reactions associated with them. Since the ones we are most concerned with are the ones that are causing the biggest negative reactions in our lives, we need the mind to be calm and tranquil in order to do this work. Concentration greatly improves this tranquility as well.
How do insights stir up Sankharas?
This would require more speculation about somatic theories that don't yet have a good evidence base. Subjectively, it feels like building up insights into particular kinds of linkages between physical sensations, feelings, and mental reactions causes areas of your backlog that are particularly heavy in those linkages to get some activation and thus be available to consciousness.
You've experienced this if you've ever had a conceptual insight and then spent the next week noticing ways it was applicable, seemingly spontaneously. The only difference here is that insight can also be non-conceptual (ie, insight into how two particular physical sensations interact might generate no verbal content/mental talk but some sense of something happening.)
How does clearing Sankharas improve concentration?
The mental talk, emotional avoidance, and physical discomforts that interrupt concentration practice are built from unendorsed linkages.
So, the Buddha taught a method of concentration, a system for developing insight that we know as mindfulness, and to use these to both stop building new stories and to clear out our backlog of stories. That's actually it. The rest is details for how this plays out in practice. Failure modes can get a bit weird, and even if you do it right some mind blowing states and experiences can pop up. So there's lots of whataboutism for all that.
The miswired central nervous system story gives us simple answers to things like trauma (extreme levels of miswiring of things into fear and freeze responses), why stuff like yoga and exercise help (general CNS health, probably capacitance/fuse breaker improvements), why psychotherapy sometimes but not always activates childhood memories and the significance of that, and why practitioners claim they have a much better life but can't always explain why (they perform the same actions but with much less internal resistance).
So then why all the rest of this crap?
Well, besides my post on why practitioners make so many metaphysical claims, it's also just that there's a lot of idiosyncrasy in first unwiring a randomly wired CNS and then rewiring it in arbitrary order. Especially when you don't really know that that's what you're doing as you're doing it and your mindlessness teacher is a bit clueless as well (though may still have good pragmatic advice despite bad epistemics.)
In addition, each of the practices is actually a practice category. Though the Buddha taught one specific concentration technique and a simple series of insight techniques, but there are probably a dozen alternatives in each category that seem to work for some people and which entire traditions have subsequently built themselves around and gotten into fights with rival schools about.
Note: I am fairly confident this is how things work up until 2nd path. Since approximately zero percent of people make it beyond that point I'm not too worried about this.