The official Python installer in the Windows App store installs a handler for python and python3 .. or rather, the stub of the handlers already on your system calls into the app store and it's behavior changes after installation. Regardless of the technical implementation, after you install Python3 from the app store, running python3 from the command line will open the Python interpreter.
No clue why the standard non-app store installer doesn't add python3 to the path to mimic this behavior.
The standalone installer is quite old and frankly weird. You're meant to use `py -3` or something like that. I don't know why it works this way. Tbh I think the official store installer works much more sensibly.
Back when I had a large python2 and python3 code bases side by side, I had a small handler that I installed for .py files that read the #! line and tried to figure out which python version to call, and defaulted to Python3 if it couldn't find a #! line. I always felt this was the "correct" way of solving this.
Nowadays, I just only install Python3 on machines, so it's not really a problem for me anymore.
No clue why the standard non-app store installer doesn't add python3 to the path to mimic this behavior.