.NET is a large collection of a bunch of different things really; including compilers, runtimes, programming languages and a bunch of different tools and technologies.

It provides a lot of common functionality that can be tapped into rather than recreating it from scratch.

C# is one language that can be used on .NET. It get compiled initially by Roslyn and can get additional compilation at runtime by RyuJIT. Other languages include F#, which can all be compiled and run against its runtimes.

This works because it’s generally not compiled to a particularly low level language but rather to an Intermediate Language that can be executed by a .NET runtime. The runtime then does the appropriate translations to actual system/hardware calls allowing you to not have to worry about it.

When you create at least C# projects, you have to specify what runtime you’re targeting so that it can be compiled appropriately to work; not just whether it’s Framework or Core, but also the version number since functionality of the same thing may be different.

One of the goals with .NET 5+ is to reduce the confusion by bringing many of the parts together under one umbrella, but it’s going to be a while before that replaced enough existing software to really get rid of the confusion (especially since .NET 5 isn’t going to have Long Term Support).