.NET Framework + C# fits the bill. Twenty years old, but still feels clean, modern, and reliable as it continues to evolve in a coherent manner.
In my opinion, it's hard to identify the best entrypoint to modern "boring" C# because of all the different subsystems: Core, Framework, ASP, etc
C# and .NET (6/7/8) is what you may be looking for. The entrypoint for that would be the basics[0] (just pick in the left column whatever strikes your fancy, the official docs are good) and then if you are interested in back-end dev - ASP.NET Core[1].
[0] https://learn.microsoft.com/en-us/dotnet/csharp/tour-of-csha...
[1] https://learn.microsoft.com/en-us/aspnet/core/fundamentals/m...
The rough overview of terms you have mentioned is as follows:
.NET (6+) - the platform almost everything runs on, like OpenJDK but encompasses the standard library and certain nuget packages as well
ASP.NET Core - main .NET's web framework for developing web applications like LOB CRUDs, gRPC-based microservices, etc. Doesn't really have alternatives that offer comparable performance/features ratio
EF Core - main .NET's ORM framework, its popular alternative is Dapper which is more hands-on and focused on writing SQL code directly
Notable mention: .NET Core used to be the name for .NET Core 1.x, 2.x, and 3.x version but was later on rebranded to just .NET. The version 4 was skipped to avoid confusion with .NET Framework 4.x which will live on forever by the nature of being included with Windows.
I would personally recommend .NET for backend work, as MS track record of UI frameworks is rather suspect. For the main project I currently support, using .NET 6 API and a React frontend (+EF Core to access a Postgres database). Works beautifully.