Introducing .NET Core 3.0
This part of the book is about the functionality in the APIs provided by .NET Core 3.0, and how to reuse functionality across all the different .NET platforms using .NET Standard. .NET Core 2.0 and later's support for a minimum of .NET Standard 2.0 is important because it provides many of the APIs that were missing from the first version of .NET Core. The 15 years' worth of libraries and applications that .NET Framework developers had available to them that are relevant for modern development have now been migrated to .NET Core and can run cross-platform on macOS and Linux variants, as well as on Windows. For example, API support increased by 142% from .NET Core 1.1 to .NET Core 2.0.
.NET Standard 2.1 adds about 3,000 new APIs. Some of those APIs need runtime changes that would break backwards compatibility, so .NET Framework 4.8 only implements .NET Standard 2.0. .NET Core 3.0, Xamarin, Mono, and Unity implement .NET Standard 2.1.