Need some guidance regarding what to do in Backend next??

Hey Backend devs & seniors,

Recently I built an app with basic auth and simple REST APIs.
Basically a web extension which sends data to frontend with some mongo aggr and login register and change password auth.

Now I want to go deeper into backend.
I'm starting with PostgreSQL and TypeScript. What resources, projects, or concepts should I focus on?
Any tips you wish you knew when you were starting out?