You do not "memorize" it, it's too big - and by the time you know it by heart, there'll be a new version. You'd learn how to use it; once you worked through a C#-book, you should be able to use the documentation[^].
I'd add that when I started out I invested in a copy of the Base Class Library text (it was c# 2005 at the time). That was a great book for showing me a great, useful slice of the framework, how it works, and what it can do.
Once you get in the 'mindset' of the framework, how it goes about things etc. You can, more often than not, predict where in the documentation you'll need to look to solve your problem.