Customizations are something you can buy with gold. Currently you can only customize the player, which includes the following: hair style, face, torso, shoulders, hands, belt, legs, feet, and flight aura. Weapon customization is said to be in the works.
It was originally planned back in early 2020 that customizations would include "different clothing styles, capes, bling, particle trails and other stuff...", along with "allowing colored hair", though he went back on this, stating that the colors would be limited so as not to confuse people about bandana color. So far, only the first part of this has been rolled out, and likely will be the only one to be rolled out.