[vog]

I'd recommend starting "from scratch" for the simple reason that the official PostgreSQL manual is excellent:

https://www.postgresql.org/docs/14/index.html

You'll quickly find which parts of the manual are interesting to you and which ones just repeat what you already know (or could have guessed on your own). Since the manual is very well structured, skipping those parts is very easy.

[JeffRosenberg]

Look at that, there's an entire section on internals. Honestly, I didn't even consider looking there, I just assumed it would be all about syntax and never bothered to check. Thanks!

[Tostino]

The Postgres docs are great because of that. They are kept on top of pretty regularly compared to most open source projects, and document things in an understandable way.