I recommend Postgres FM podcast, e.g. available as video on Postgres TV yt channel. Good content on its own, and many resources of this kind are linked in the episode notes. I believe one of the authors even helped Gitlab specifically with Postgres performance issues not that long ago.

I like this, although a bit narrower - https://blog.mastermind.dev/indexes-in-postgresql

Here's another - https://zerodha.tech/blog/working-with-postgresql/