ECMA spec is for the CLI, essentially the specification for runtime like type system and GC and all the other things that make an environment work at all

Net Standard is a specification of APIs. So in order to be compliant to the standard you must have a class named X with parameters string Y, int Z.

"Net Framework" is the name for the Windows "full .net framework" -- this doesn't have a descriptive name since it was before all the rest, but basically its an SDK that is a superset of the Net Standard (and therefore is .Net Standard compliant / interoperable)

Mono is a open source implementation of "Net Framework" that runs on multiple platforms (linux, etc). It attempts to treat full .NET Framework's surface area as a spec but re-implement it cross platform. It is _also_ a open source CLI implementation. So in common speech "mono" like ".net" can refer to either the "basic sdk" _or_ the runtime environment.

Xamarin is forks of mono for iOS, Android and Mac platforms plus libraries that include full support for interacting with those platforms Native SDKs

Unity is a fork of Mono plus gaming tooling and runtime tools. Microsoft does not maintain this, it's a separate company.