The section on parallel joins is a bit out of date. Shameless plug: I've written and spoken about this subject extensively, if you are looking for more information, with further links and pointers to other blogs and academic papers.

Parallelism in PostgreSQL in general:

https://speakerdeck.com/macdice/parallelism-in-postgresql-11

Deeper dives on hash joins:

https://write-skew.blogspot.com/2018/01/parallel-hash-for-po... https://speakerdeck.com/macdice/hash-joins-past-present-and-...