FYI, You are comparing the modern version of DotNet (the first link) with the old legacy version (the second link).
The modern version of DotNet, "Net Core" is effectively a reboot of DotNet, with a very cross platform focus and redesigned API's based on decades of experience.
The impressive thing between .NET Framework (original .NET) and .NET now (rebooted as .NET Core but now dropped the “Core”) in that they largely foxed the API while leaving almost all of it intact.
Library code you wrote in C# 10 years before .NET Core will often just compile and run. Even more than code the resides developer learning. The plumbing between ASP.NET MVC (old) and ASP.NET Core (new) was completely and radically different. Yet writing an application in it was very much the same.
The first link appears to be for .NET Standard, which has a common API compatible with both Framework and Core.
Though it might be worth checking Github to find example usages of the APIs. Maybe there's even some libraries that improve the developer experience with cryptography.
The modern version of DotNet, "Net Core" is effectively a reboot of DotNet, with a very cross platform focus and redesigned API's based on decades of experience.