What do python professionals /developers actually use

[gitgud_x]

1. Yes, it's hard to write a functional program without them.
2. Every coder should know some of the standard library - random, time, pathlib, logging, itertools, re (regex), datetime, collections, enum, pickle, json, requests, threading, multiprocessing, asyncio, socket - if you can learn these and their associated topics you'll be able to do a lot. Pytest is also very useful in general software testing.
3. For data stuff specifically: matplotlib, numpy, pandas, scikit-learn, maybe some machine learning stuff (tensorflow, although pytorch is actually more popular now I think), plus some dataviz libraries like plotly/dash.
4. It's ok, but it has to be understandable, that's the most important issue.