I have read about the difference between .NET Standard and .NET Core, but I really don\'t know what the difference is, or when to choose a .NET Standard library project and
.NET Core is an implementation of .NET Standard. It's available on multiple operating systems, but that's not the same thing - there are other implementations of .NET Standard as well.
So if you create a .NET Core library, it will have access to things that are implemented in .NET Core, but aren't part of .NET Standard, and your library won't be compatible with other implementations of .NET Standard, such as Xamarin, Tizen, full .NET desktop framework etc.
In short: to achieve maximum portability, make your library target .NET Standard.