Pytest archtecture question - trying to learn pytest - fixture closure

[Tusenka]

Intro:
Hello)
I try to learn pytest code to contribute.
Question:
Is there any scheme or architecture to this? From which file should I start?
In partiuclar I try to perform understanging where ._arg2fixturedefs has been added for FixtureManager.
fixturedefs = self._arg2fixturedefs[argname]

[Tusenka]

Found it here
parsefactories but can't understand how it works