Unfortunately mastery in programming is often an ongoing process, particularly in languages in C# where development rarely slows down. Having said that there are resources that are always worth going through that'll speed up getting to the level you want to be at:
- Get a basic familiarity by reading through documentation: https://learn.microsoft.com/en-us/dotnet/csharp/
- Reading the devblogs for new features (you'd be surprised how after professional developers neglect to do this, so it's definitely beneficial to keep up to date): https://devblogs.microsoft.com/dotnet/
- Understanding common programming patterns: https://refactoring.guru/design-patterns/csharp
- Looking through a lot of other people's code and slowly trying to gain an intuition for what large C# projects look like and why they look like that
- If you're using a particular API inside C# and you want to understand how it works, you could have a dig around inside the C# reference source: https://referencesource.microsoft.com
- Maybe you want a more holistic view of .NET as a platform - in which case you could clone some of their repositories and have a root through the bits that interest you: https://github.com/dotnet
- Reading through topics that come through forums like this one also has helped me keep an eye on what's going on in domains that I don't normally venture in for my career