I'm not sure if this is what you're looking for, since its more at the level of data and psychology than neurology, but I'd strongly recommend reading A Human's Guide To Words. Most of your question - "How do humans understand what words mean?" - isn't specific to text, and applies just as much to spoken language, sign language, and any other form of symbolic communication that uses the brain's native language capabilities.
At some level, language is always metaphor, where meanings are inferred by cache lookups searching only a few steps away in a very highly branched tree in concept-space. It has to be something like that. Poetry is just one or two more inferential steps removed from "literal" meanings, so we read it a little slower and still miss some of the richness of how all the concepts relate to each other.
I do wonder how much interpretation of written language specifically varies from person to person and language to language, but that's a separate question about how the brain relates written squiggles to the concepts it already has stored as "words." For me, I read aloud in my head, and thereby connect letters to spoken words. This... did not work for me when I was taking Mandarin classes and reading characters. Too many homophones. My teachers told me they never do this. For them, characters relate directly to words which have meaning, and spoken syllables relate to words which have meaning, but neither "are" the words. I don't know how common their experience is for anyone else speaking any language. I would be curious if deaf people reading English experience something similar, and if deaf people have an easier time learning to read character-based languages or less inflective languages. Or if illiterate people process homophones differently.
What are the neural/brain faculties in humans which enable us to read and understand text/messages/thoughts of others by reading and unconsciously processing them whilst inferring their meaning from what we already know ? (knowledge of the language being processed, the usual/generic meaning of its words as well as the point of the question/topic is understood)
Especially poetry, adages "Little strokes fell great oaks." and useful and insightful texts which doesn't actually state/give direct and blunt advice in the spirit of "Do not overthink every decision or move you make, both in everyday tasks and specific projects , just do it with full force and witness/bear the results." kind of advice versus something like this - Gödel's Legacy: A game without end. That is, how do you (fellow humans, don't know about ML NLP or whichever way machines use to understand human-generated text) understand what I am asking you despite me not being more direct or cryptic with my written request?
Edit: Changed the title from "The Crux of Intelligence and Understanding - Text reading and inference" to "The Crux of Understanding Written Text - Text reading and inference" <- I do think this ability alone is the most advanced faculty of learning about the world in general and thinking about it with goal-seeking motivation in mind is the pinnacle of human ability and intelligence. Feel free to correct me/criticize me if I am overstating or if this kind of thoughts generally get on your nerves. :)