As Ben wrote, some cycling skills can only be practiced outside.
But there’s plenty you can learn on Zwift, inside. Seemingly basic stuff, from sitting on the saddle with the saddle set at the right height, to pedaling smoothly, to changing gears to keep your pedaling cadence smooth as you go up and down the terrain.
And then there are pack skills, riding around other people. Real world riding isn’t exactly like Zwift, but learning to keep up on a Zwift group ride is useful.
Some groups I’d recommend you to look for are the Herd, ZER, and ZSun.
You can look for group rides easily at https://zwifthacks.com/app/events/
Below are some examples of using a filter at Zwifthacks to find some possible group rides. To start with, look for “D” rides. Another indicator is “sub” in the title. The groups that follow their advertised pace (not all do) will ride around 2 watts of effort per kilogram of body weight, or 1.5, or 1.2, etc. Obviously, the lower the number, the more suitable it is for people just starting to build cycling strength.