To make stereo track out of a mono, I used to be able to route a mono track straight to master. Then, on a stereo track, I used to be able to select MASTER as an input option. Then I would play and record on to the new track as stereo. It used to be fast an easy.
Now, the option to select MASTER is no longer there.
Cubase 9.5 had the ability to never let a continuous “ground loop” be select so it would never let a track feedback into itself.