This part was always magical. It seems to free up the local memory usage of the program, but it doesn’t seem to release the memory back to the operating system, until some type of triggered event.
That's up to the C runtime's memory allocator. Modern memory allocators don't typically request a new chunk of memory for every malloc() call -- instead, they allocate a single large region of memory at a time and carve that up as needed. This is massively more efficient (system calls are expensive), but also means that those regions can't be released to the OS until all allocations in them are gone.
The Python/C Reference Manual has a great section on memory management [3].
[1] https://docs.python.org/3/c-api/typeobj.html#c.PyTypeObject....
From this, I guess CPython opted for simplicity instead of implementing something like a memory-usage monitor.
[0] https://realpython.com/cpython-source-code-guide/#garbage-co...