Help for my first built

Hi ✌🏻

I live in Berlin (Germany) and got this bike for 50€. It already rides really nice, and it’s already my main commute companion.

As I said in the title is my first rebuild, I am new to this so I would really appreciate any advice or help for:

• what should I learn first?

• general advice on what to modify

• cool pieces to add/change (EU/germany available please)

• shops here in Berlin/websites where I can buy good quality pieces

• anything that is not expensive but looks neat

Thanks 🙏🏻