I want to create a backend app in Python. Should I use a third-party library? How does the code looks like?

Answer 1

Simpliest way is probably with Django, not more difficult could be Flask, and few more tricky is Bottle: you can find a wide choice of web frameworks for Python, among which you surely will find one corresponding to your tastes and your motivation to build more or less from scratch ;)

Answer 2

You can learn Django which provides a ton of libraries for backend development
with python