The official documentation is pretty damn good. Most of the time when I needed to learn something about Postgres, it had all the answers I needed, and in rare occasions it didn't, I used Stack Overflow to figure out what other part of the manual I needed to read.

Checks posts of this author https://habr.com/en/users/erogov/posts/

he doesn't write often, but it's advanced technical topic, I find it super useful and well explained.