The bootstrapping path does not exist. There is (afaik) no way to go from C compiler to working dotnet environment. You are supposed to just download binary blobs from m$soft.
https://github.com/dotnet/dotnet exists for "complete" source build that stitches together SDK, Roslyn, runtime and other dependencies. In fact, it is required by certain Linux distributions for publishing in their feeds, to be built from source in full. All components above can be built and used individually (usually), which is what contributors also do. For example, you can clone and build https://github.com/dotnet/runtime and use the produced artifacts to execute .NET assemblies or build .NET binaries.
Thanks for the information. It seems to contain only versions 8 and 9, so I guess what I said was valid only for the previous ones.
The repository looks promising, however the build.sh trying to reach to the internet during the build is disappointing. I would expect that to not help with having reproducible results. I need to look into how distributions approach this.
Domains that require network-isolated builds usually maintain internal mirrors with corresponding dependencies. It is unreasonable to expect from a project of this size to have all tooling it depends on to be available within repository files (moreover, it should build on multiple ISAs and OSes). I doubt you can build LLVM this way or, let's say, OpenJDK.
Well for example golang builds offline just fine, which seems similar in scope? Runtime-based language targeting multiple architectures and operating systems.
And .NET can bundle the runtime, even in a single binary if you prefer that.