Yes, You should set your FTP first … either You know it or You’ll do some FTP test - see 1 in my picture
And yes, it will adjust workout values and difficulty just for you …
You can create Your own workouts or even modify current one (copy and then change - see 2 in picture). New, updated one will be located under Custom Workouts - see 3.
I modify workouts sometimes as I find some of them too hard for my current fitness … not too much Wattage actually, but have to shorten some intervals a little bit …
As I run Zwift on AppleTV and its remote control is absolute Pain in A**, I’ve installed Zwift on my Mac Pro. I use it just to configure my avatar (jersey, socks, etc…) and/or to work on workouts - change, create, etc … while I still run Zwift from AppleTV … those custom workouts are synced between my MacPro a AppTV automatically …
Hope it helps …
GdF
Edit later - almost forgot to mention … also You can change the difficulty of workout “on the fly” … if needed (as sometimes you feel stronger or weaker little bit, then usual) … there is BIAS control in Workout mode … You can adjust temporary workout difficulty +/- 10% …