I don’t feel that answers my question at all. How does this work in game mechanics? What is the structure, if any, of thought in the game.
D&D is primarily a combat game. 5e wants mechanics for social interaction and exploration, but still, mechanics of a mindscape are probably gonna be combat mechanics.
With regard to the setting concept, the astral plane is a realm of thought, something like a Platonic realm of ideas, or a Jungian collective unconsciousness of archetypes. The concepts and symbols and assumptions that a culture uses to organize day-to-day activities become semi-autonomous realities. It is something like a novel becoming a reallife twilight zone. But these stories tend to be deep stories, culturally enduring narratives, that make "reality" make sense and feel true. For example, pretty much all cultures value and depend on the roles of mothers, so in a realm of thought expect to encounter some form of mother archetype. Adventurers who journey thru these thought scapes confront various cultural archetypes, for better or for worse.
The archetypal constructs from particular cultures occur in the "astral dominions". The "astral sea" is more like empty space having a dream. The "astral wildspace" is more like the sun and planets of a solar system having a dream. But dominions are more often about human dreams or the dreams of various creature types.
Note, many of the dominions organize by an ethical alignment, NG, N, NE, etcetra. So often various archetypes from unrelated cultures sometimes get clustered together, because these archetypes share the concept − the "idea" − of a comparable ethic in common. Things are "near" to each other, when they have many cognitive associations in common, and "far away" when they have little or nothing in common. Think of alignment as an information tag, and the astral plane as "sorting" its information database to assemble all of the archetypes that have the same or similar alignment tag into the same dominion.
But the astral plane can alternatively "sort" the data according to a different tag. Maybe a dominion is all of the symbols from a particular culture, like Brasil or Japan. Then the adventurers experience these clusters of encounters because they share a nationality in common.
The thought scape is fluid. Things organize via linguistic (including semiotic) associations.
Journeying thru the thought scape, is more like trying to remember something that is on the tip of ones tongue. One might find something specific difficult to remember, but then relax, think of something else, then actually arrive at the memory via a different path of cognitive associations.
Regarding "space". There is no space. This is easy to understand. I guess that everyone reading this post has played a video game. There is no space. It is just electrical bits and bites of a computer to track data. No matter how "real" the computer screen might feel − everything is just information constructs.
Similarly, a dream. One can dream vividly about flying across the land or meeting a friend in a house. But everything is information constructs happening inside ones own skull. There is no "space". Pure information.