If you are not new to C#, start building an application end-to-end/a classic n-tier application, right from using a database, Entity Framework, WebAPI, DTO, AutoMapper, and a front-end (your choice - Angular/React/Vue with TypeScript or better yet - Blazor).
You will not only understand & use dependency injection, reflection, LINQ, multi-threaded programming, generic programming, source-code generation, unit-testing - and much much more in a practical/real-world usage.
Peruse https://referencesource.microsoft.com/ once in a while. 'Use the source, Luke'
Follow various team members of .NET Framework, C# team and the like on various social-media, StackOverlow and the like. .NET Team members >> https://mobile.twitter.com/i/lists/120961876
I am sure there will be who agree / disagree with the above approach and will provide more viewpoints for you to consider.
Enjoy - and strap yourself for an amazing journey or roller-coaster ride, however you want to call it!