I’m a mobile app developer, I want to learn something so that I can develop backends.... databases, rest apis, admin panels etc. I know basics of php, meanstack and django. What should I choose...

Build simple MVC systen, these are almost always in guides of frameworks anyways and they usually show good practices + patterns

I'd try out Symfony or Laravel, and if you're comfortable with a bit of DevOps check out some basic Docker setups :D

Php: laravel, lumen, any kind of framework, cakephp is the most noob friendly

Javascript: MEAN stack all the way.. nodejs, express, mongodb..

Python: django, flask

I come from fullstack framework background, so jumping into microframework for quick api building, ill go for lumen, flask or express js. Admin panel, you can quickly fire up django or rails, even bake with cakephp. 😅