About a month ago, Microsoft announced that it would be releasing an ARM64 SDK at its Build 2018 developer conference. Today, the firm announced that the SDK is indeed available as part of the Visual Studio 15.8 preview.
Previously, developing a UWP app only allowed for developers to compile a package for x86, x64, and ARM32, as even 64-bit Windows phones used a 32-bit OS. Now, Windows 10 on ARM PCs run a 64-bit OS, so they can support the extra bits. Yes, these PCs can run emulated x86 apps, but those are also 32-bit.