PEP 514 is the way to find it on Windows (you should be able to look directly for your supported version, rather than enumerating to search). As far as I know, the best way to find it on macOS and Linux is to search PATH for "python3.X".

Appreciate the size concerns. The Python runtime comes down to about 10MB with some basic tricks (already done on Windows in the embeddable distro), such as excluding the docs, test suite and zipping the standard library. It's possible with different tricks to do the same on other platforms, so you may even find the best balance for yourselves and your users is to repackage those and do a download-on-demand, rather than looking for a system installed Python.

> as an installer, it should be the most forgotten part of python that usually runs for once or twice. 

Oh I wish this were the case! Installers should be forgotten, but they need to be 100% reliable or people *really* notice :)

But updating the installer build to target ARM64 while generating an x86 executable should be possible, it just hasn't been done yet.