Mono is still supported. .NET core and Mono solve different problems - for example (this surprised me), unlike Mono and the windows .NET Framework, .NET core can't just run random .NET executables you download off the internet. You have to build a core-targeted project from source. Mono also handles cross-platform scenarios like mobile & mac frontend apps right now that .NET core won't be a good fit for in the near future.
Mono, .NET Framework, .NET Core, Xamarin.<Platform>, etc are all just different implementations of .NET Standard. I believe .NET Framework is a superset of Core.
They can all run code targeted at Standard but all have slightly different libraries on top of Standard. Mono was originally was intended to be a cross platform stand-in for .NET Framework which is why they can mostly run the same code. I believe Mono and Framework are slowly merging together.
I don't think this is true. Framework for example can only reference standard libraries as of 4.6.2. Up until dotnet core 2.0, core was very much an incompatibile subset of framework - even with 2.0 it would be better characterised as a cross platform rewrite of the net framework.
