How to open Python?

[snippsat]

I can try to give some getting stated tips.

"Once in shell of your choice cd (change directory) ...." That is alien to me but I may understand it better after I investigate your links.

This is a little worrying as you done other stuff for a while and not learned command line better cmd tutorial .
Command line usage is very important in programming,i use cmder which is lot better,you can look at it later.

Start:
Python 3.9/3.8 and pip installation under Windows
Try follow this and eg install Python 3.9.5 and run command and code from cmd.
It's important that python and pip commands work from anywhere in cmd(Then OS Path is ok).

Editor:
Here would i suggest one that is very simple to use and also good editor.
PyScripter

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And why are the tutorials not describing how Python should be integrated with HTML & CSS?

There are many out there,get the basic to work first.
In Python is Flask and Django the biggest.
Flask is more similar to basic PHP programming as you close to HTML/CCS.
Have tutorial here basic setup Flask and Flask]Weather app
And very good Editor for Python(alone) or when programming with web-development
VS Code from start