.NET 5 unifies Mono and .NET Core, not .NET Core and Framework. Previously Mono and .NET Core had different base class libraries and machine code generators (Mono using LLVM).

Mono is an implementation of the .NET Framework. While the code of v5 might be based on Mono and Core, v5 is also unifying the APIs between Core and Framework.

Most of this is already true because of .NET Standard 2.0, which latest of Mono, Framework, and Core already implement. Large swaths of the extant OSS libraries for .NET have already converted to .NET Standard 2.0. There are mostly only niche use cases left that today that you can't build into .NET Standard libraries and import into programs compiled for any of the extant CLRs.

My understanding is that .Net 5 is a new version of .Net Core and that the .Net framework is dead.

Microsoft naming has been executed poorly.

If you don't have daily familiarity with these tools, they are absolutely inscrutable.

Correct. This is also double trouble when Microsoft changes the names of the same product for a future release.

If you actually use the tech, it's really not that confusing