How can i get this effect (2d motion)

Hello,

Here is link to animation made by Thomas Hedger

Strangers by Late Night Work Club | Animated Short Anthology (check time 36:20)

Character shirt waves constantly on top and above legs.

Are those two layers? Or some effect?

Im looking for solution, how to make something pretty similar to this.

Here is screen showing the wavy motion

It would be super cool if you guys have any tutorial

Project file with solution would be more than helpful

Thanks!