What concepts I should learn to master C# ?

Ik the basics and I have learned these concepts and I solved labs about them (besides I learn unity at the same time)
-basics ofc
-OOP

-Delegates

-Events

-Generics

-LINQ

What I should learn next and what projects also I need to make ? I made some projects in unity but I stopped unity for a while to focus on pure C# besides I want to lean asp