yes, post_read marks which posts a user has marked as read. it links post ids with user ids and adds a timestamp on top to allow for sorting.

edit:

for comments, lemmy only stores the number of read comments per post, which is what goes into person_post_aggregates. this is a tradeoff, it has some limitations, e.g. when comments are deleted or removed, which affects the total count. as there is also a timestamp attached it might be possible to use that in combination with comment creation times, though this would likely impact query performance quite a bit more.