That depends, if this is your first plane you learn a lot of general flying. However if you can already fly all you are learning is how this one airplane is different. In an emergency you need to know how your plane will react as you need to have the right reactions for trouble which is different in different airplanes, so if this is your second plane there isn't nearly as much gain.

Likewise, if this is your first military jet you also learn fight tactics. However if you have already flown a military plane you know that. Here there is something to learn though as there are some NATO standard controls that would apply to other planes but not the airplanes they have had before . (Ukraine has rigged up some air to air missiles to work from MiGs, but with proper integration there are controls Ukraine hasn't had the ability to use before that they now get to learn.