See, here's your problem: you know too much
For the moment, forget about UV maps. Let's just talk about appliers. They are their own HUD, they don't show up in the body HUD per se. I recommend you pick up a few free ones. For example, MMC has some applied bodysuits etc. as free group gifts. They're also available
on the marketplace for $L10 each. When you click on the applier HUD, it will somehow present you with a choice of which of the 3 layers (generally referred to as tattoo, underwear and clothes) you want to apply to. Note that some items, for example a catsuit, will require you to apply to both the torso and lower body, so you would (for example) click twice to apply it, once for the upper body clothing layer, once for the lower body clothing layer.
Oddly, you cannot remove applied clothing via the HUD you used to apply it; you use the body HUD. So AFTER you've applied something, open up your body HUD (I'm not familiar with Belleza's) and find the tab for the onion layers. You should now see some indication reflecting what you just applied. You should be able to REMOVE it by either just clicking on the layer or a related button. Hands and feet can be funky to remove even if they were applied as part of the garment, so they sometimes have their own buttons in addition to the lower body or torso button. You may also see a way to turn alpha masking on or off for a layer; sometimes it is necessary to turn masking on for a lacy texture or something.
It looks like those Myra leggings come with BOTH a Belleza applier and an Omega one. You don't need the Omega applier if an item comes with a Belleza one, and nearly all applied clothing will have a Belleza applier.
If you have your own clothing textures, rather than messing with the Belleza kit, it is easier to set them up as Omega appliers. You'll need this:
https://marketplace.secondlife.com/p/Omega-System-Kit-Belleza/6412085
I think that package will contain both the Belleza Omega relay HUD that you need to wear whenever you're using an Omega applier, AND instructions for how to build your own Belleza Omega huds. All you need is the UUIDs of the clothing textures you want to use, and usually any old regular SL clothing textures will work fine, no need to mess with maps.
If you want to make your own skins, makeups, etc. you may need to mess with maps. You can probably get away with using the SL UV maps to get started, most bodies are derived from them.
