When running python (cpython) on windows the <built-in function system> os.system will execute under the curtains _wsystem while if you're using a non-windows os, it'll use system.
On contrary, Popen should use CreateProcess on windows and _posixsubprocess.fork_exec in posix-based operating-systems.
That said, an important piece of advice comes from os.system docs, which says:
The subprocess module provides more powerful facilities for spawning new processes and retrieving their results; using that module is preferable to using this function. See the Replacing Older Functions with the subprocess Module section in the subprocess documentation for some helpful recipes.