I can find partial information on how Python treats __main__.py here: https://docs.python.org/3/library/__main__.html

However, it is not documented how python handles __main__.py when passing the Python package to the interpreter without -m. If I have a program defined by /tmp/foo/bar.py and a file /tmp/foo/__main__.py, and I run

python /tmp/foo

... then Python will run __main__.py, with /tmp/foo prepended to sys.path.

I find this behaviour unfortunate; to me it would make more sense to prepend /tmp to sys.path, because then "python /tmp/foo" would be equivalent to "PYTHONPATH=/tmp python -m foo". With the current behaviour, if __main__.py wants to import bar.py, then it must write 'import bar' or 'import foo.bar' depending on whether the interpreter was invoked with -m.

Unfortunately, by Hyrum's Law, you can find people describing and encouraging reliance upon the current undocumented behaviour, e.g.:
https://www.geeksforgeeks.org/usage-of-__main__-py-in-python/
so perhaps the behaviour can't be changed that easily. Therefore, my request is to document how it works.