I would highly recommend talks by Bruce Momjian https://momjian.us/ he has separate talks on pretty much everything related to PG + All The Dirt On Vacuum talk by Jim Nasby https://www.youtube.com/watch?v=L8nErzxPJjQ

[0] Online documentation - Internals - https://www.postgresql.org/docs/9.6/static/internals.html

[1] Wiki - https://en.wikibooks.org/wiki/PostgreSQL/Architecture