Need to upgrade legacy .NET Framework apps to .NET Core? This article goes over the big changes between the old and the new along with examples of commonly encountered issues and ways to mitigate them. Also a link to an entire free eBook (Porting existing ASP.NET Apps to .NET Core) [PDF] as well.

https://docs.microsoft.com/en-us/aspnet/core/migration/proper-to-2x/?view=aspnetcore-6.0