How do I know what I need in C# or another language like c++

Answer 1

It depends on your purpose.
You need to clarify what problem you are going to solve.

Break the problem into small pieces and check them all.
Then you may find what you need.

Answer 2

https://code.sololearn.com/W7X805sc4DPC/?ref=app

Answer 3

did you mean learn from open source like GitHub website