Ah, Trainer Difficulty: The great debate of Zwift.
IMO, trainer difficulty is NOT about gearing, it is about gradient. That is, it doesn’t really affect the virtual gearing on your physical bike, it just affects how Zwift translates the virtual gradient into resistance on your trainer (presuming you’re using a smart trainer, that is). This, in turn, affects the actual gearing you are able to use to ‘climb’ or ‘descend’.
So, if you use the Zwift default of 50%, a 10% climb feels like a 5% climb, and you are able to use the gearing you would use for a 5% IRL climb. But it also changes a 10% descent into a 5% descent (or, really, a 2.5% decent, because Zwift is already halving the descent gradient internally), so you won’t coast as quickly going down. (This is why some people notice that they coast to a stop on a Zwift descent when on the same gradient IRL they would continue to coast down the hill.)
So, you can’t really use the trainer difficulty to change your bike from 9-speed to 11- or 12-speed, but you can use it to ‘flatten’ the terrain and make it seem like you have a wider range of virtual gearing.
(This is my take, at least. I’m guessing others will chime in with some alternate views.)