how do you come up with cool outfit designs like this?

Lots and LOTS of research, this; admittedly is on the upper end of designwork, I dont think my wrists will be kind to me if I keep having to redraw this.

Oh and this one in particular, look up Granblue Fantasy Relink; definitely took a lot of elements from there.
Unfortunately (fortunately?) Gacha games have very good clothing design, its meant for mass appeal so there's good stuff to learn even if you (I) dislike gacha games!