If you are making a Win32 app that uses Fido then use the WebAuthn.dll API.
If you want to develop a library that acts as a Fido2 client and talks to security keys and the platform authenticator, like libfido2 then you can, but that lib on Win10 uses WebAuthn.dll to avoid needing to run as Admin to access the Fido HID device.
UAF is less common on Windows now that Win 10 has Fido2 support and a built in platform authenticator.
You could build a UAF library for Win 10 but you are probably about 5 years too late for that having a real market.
I hope that helps.
John B.