Regarding the multivitamins, since you said "more than you wanted to know", if you have the time it might be worth taking a look at something similar to these Nutrient Reference Values and comparing with the amount of the nutrients in the multivitamin: https://www.nhmrc.gov.au/about-us/publications/nutrient-reference-values-australia-and-new-zealand-including-recommended-dietary-intakes
I had heard that multivitamins are mostly useless (but I can't remember where) because it is impossible/expensive to physically fit everything into a pill. After looking up the recommended intakes, I noticed that many of the quantities of each of the vitamins in the multivitamins I looked at were basically negligible compared to the recommend intakes. I stopped these multivitamins and started taking a few specific vitamins similar to those you mentioned. As you said, I have no idea if they are achieving anything.
Those recommended intakes are specific to Australia/New Zealand, but I suppose many countries must publish their own so you could pick one you trust. It does seem to be a theme that government health departments often don't recommend supplements (e.g. https://www.healthdirect.gov.au/vitamins-and-minerals).