I agree that the doc needs more, but I am closing this as a duplicate of #31329, which is specifically doc about starting IDLE.  You can still answer Paine's questions here if you want.

File association: IDLE is not Python.  It is one of many Python-oriented editors and IDEs.  .py files are and by default should be associated for running with something that runs the file with python.exe.  On Windows, this is done via C:/Windows/py.exe.  The default version for double clicking is determined by a checkmark in the installer.

The Windows installer does associate .py files with IDLE for editing: rt click, edit with IDLE ....

idle.exe is not needed for starting idle.

I don't know what you mean by 'source format'.  IDLE is written in Python.  The directory structure is mostly implementation detail not relevant to using IDLE.  File are described in idlelib/README.txt.  This might be mentioned in the doc.