System.Graphics is a cross-platform graphics library for iOS, Android, Windows, macOS and Linux completely in C#. With this library you can use a common API to target multiple abstractions allowing you to share your drawing code between platforms, or mix and match graphics implentations within a singular application.
Within the dotnet ecosystem there are multiple graphics libraries available depending on your target platforms; however, if you are doing cross-platform development there is not a unified graphics abstraction. Some legacy API's (System.Drawing, I'm looking at you) only have limited support/usefulness on non-Windows platforms. SkiaSharp runs almost everywhere these days, but for many use cases the native graphics abstractions are needed.
This is an experimental library; however it's based on code that's been in use in production applications for over 10 years. Because it was refactored out of another code base, some things may have been broken in that process.