What sources of information (books, articles, conferences) that reveal the fundamental difference between .NET 5 and NET4 can you recommend?

Question

.NET

.NET Core differs "under the hood" from .NET, at least in that it is already cross-platform.
Version 4 of .Core has already changed (1-3 .NET Core, NET 5).
If any source that systematized described the differences quite thoroughly: interactions with operating systems, means for working with multithreading? If there is no such source, please recommend sources from where information can be collected in parts.
I would also like to know how kestrel works in detail (I strongly suspect that it can vary greatly for different OSes)