Does "async/await" always run on a separate thread from the UI?
No.
From my research, the recommendation is to use "await Task.Run" for CPU intensive tasks and "await/async" for data transfer scenarios (I/O)
That's a good general guideline for UI applications. Plain async/await doesn't use additional threads; it just keeps your UI responsive. If you have CPU-bound code, then you do need another thread, so you use Task.Run, and you can consume it using await which keeps your UI responsive while a background thread runs the CPU-bound code.
but others have recommended using await Task.Run (if needing the return) or just Task.Run (fire and forget) for ANY longer running activities in Xamarin.
I don't recommend fire-and-forget. Among other issues, it can hide exceptions. I recommend always using await.
So what is the best use for "await Task.Run" especially in the context of Xamarin, in contrast with just async/await?
The general rule above is valid: use async/await for I/O operations, and Task.Run for CPU-bound operations. Every once in a while there's an exception to that rule. E.g., sometimes I/O-bound operations are blocking and they don't provide a fully asynchronous API; in that case, it would be proper use await Task.Run to block a background thread instead of the UI thread even though the operation is technically I/O-bound and not CPU-bound.
Further reading:
