If you're still confused about all the .NET Standard stuff, you're not alone. Microsoft has its work cut out for it clearing up exactly how all its .NET initiatives fit together, including the recently added elephant of Mono. In pursuit of this goal, the company has compiled a whopping big poster, showing how all the libraries, APIs and projects relate to each other in the .NET ecosystem.
How helpful it is depends on how neck-deep you are in .NET... but not just from the Microsoft perspective.
The poster outlines how a number of third-party endeavours factor into .NET Standard, with the likes of MonoGame, Unity and CryEngine filling out the games development side and Google Cloud and Amazon popping up in the online services department.
Even Rider, JetBrains' competing IDE for C#, makes an appearance in the tools boxout.
The middle of the poster contains a breakdown of APIs supported in .NET Standard 2.0, a topic that Microsoft has gone back and forth on. I suppose this list can be considered definitive for the most part, but I'd probably still keep an eye on the .NET repos, just in case.
I've cooked an optimised PNG version of the poster (just click the lead image for the larger size), however the original PDF with vectors can be found on Microsoft's website.