>>281626403
You can weight paint the skirt yourself to change how each bone affects the movement of the mesh but that is not a beginner exercise. Might be worth playing around with if you have the time though. It's not hard to weight paint but it takes practice and knowing what things do and how they work. As for unity you can use the extremely unoptimised way of putting dynamic bones or cloth components in your skirt and colliders in your legs but I cannot recommend this at all as it won't completely prevent clipping and also will nut all over everyone's frames. There might be another work around with joint components in unity too but I don't know much about it myself. Reparenting leg bones and weight painting is the easiest option.
This video may help as a guide to a well optimised and good looking skirt armature: https://www.youtube.com/watch?v=8u1NBstIcy8