That's .NET Native, which is a completely different beast unfortunately. .NET Core is set to add support for ARM64 in .NET 5. Furthermore many .NET apps for Windows were built using WPF, which won't support ARM64 until .NET 5.

It is compatible with .NET Standard 2.0 and the latest version makes use of .NET Core 2.3.

So plenty of stuff is available.

Besides it is not like one can blindly run .NET on other platforms, because plenty of applications make use of COM or DLLs written in C++.

Sure, and that's why a good number of UWP apps have ARM64 versions. But UWP apps represent a miniscule fraction of the .NET Windows apps out there. What makes it even smaller is the fact that .NET Native is even a subset of .NET Standard, so some things like Reflection don't work without some additional tweaking. Can you write a UWP app using .NET that runs on ARM64? Sure! But basically every non-UWP Windows .NET app needs to wait for .NET 5 to build for ARM64.