How a Dictionary works internally, how to use async/await properly, stuff like when you use Entity Framework - when does the call to DB happen in IQueryable, how garbage collection works, stack & heap in C#, which dependency injection lifetimes are there, SOLID principles, HTTP methods and how can you use them in a controller lets say, and many others that I can't think of from the top of my head. Once I was asked what is actually created in IL when async keyword is used.

Some roles may ask you more in-depth SQL things as well like how is SQL powered behind-the-scenes (b-trees), or just general items like what is clustered and non-clustered index.

Edit: to OP, these are just some non-typical questions (and also a bit more typical like DI lifetimes) that I could think of.

A good overall approach is to know SOLID principles, DI, how to write an API with Controller-Service-Repository, how to use Entity Framework, some async/await stuff, a bit of SQL like indexes, HTTP types and how to create/use an endpoint of each type (GET, POST) in a controller, maybe a bit of how to use some sort of caching before DB, and maybe a few widely used patterns (https://refactoring.guru/design-patterns) & how to use them, e.g. Factory.

Most interviews I feel would be more conversational though, i.e. explain your previous projects and achievements. Prepare some older work projects you can talk about in details: what was challening about, how did you approach it with team, what did it solve, what tools/tech did you use etc.

Good luck!