I need help to understand everytype of fit used in assemblies in plastic , how much clearance shud be used to mate the parts i am very confused please guide my path . if possible wanna go deep
Beyond the scope of a discussion IMO.
I have some PDFs in my personal laptop. I can find it for you when I am home. In general you can find some very good information in the PDF here.
Fantastic details on many types of fits.
Check this link too. The fits are thought out for 3D Printing but you can learn quite a lot from here too.