C# is really very big programming language

So

What are all the C# sections I must study for Master Unity?

Short answer: You have to master the programming language to master software using it.

Long answer: There is not really a subset of the language that you could read about and then be able to master Unity. You have to know the fundamental parts of C#, how the memory model works, difference between stack/heap, copy by value and by reference means, how and why the compiler boxes certain values, garbadge collection to understand why some code is getting slower than others. Operator overloading, class design, inheritance which data structures are there, which to use when and so on.

TL;DR you need to study everything that is not related to certain class libraries that Unity already abstracts for you or you simply don't need: like Networking, Graphics or Encryption. Reflection on the other hand is widely used in Unity Editor code