Despite all the wonderful work Microsoft has done with CoreCLR, the "IDE" debugger is still only available under a license that only permits use with Visual Studio and VSCode  and mdbg is still nowhere to be found for CoreCLR .
From what I've gathered, the best you have right now for doing command line debugging is the "SOS" plugin for lldb, which seems to require building the lldb plugin and sometimes even lldb (!!) yourself .
Not very fun if you're not using Windows.
I say this as someone who was using ICorDebug 13 years ago.
Read “The Bad News”
Also the licensing issue
Then a week later with the licensing issue still in place Jetbrains implemented their own debugger, on the comments it reads "This is our own debugger implementation."
ILSpy is a lightweight decompiler that you can just drop DLL files into to see their code. It has some nice analysis tools that allow you to find references and implementations. I associated .dll files with it so they open in ILSpy by default.