This goes into the instructions for how to create the characters/art.
You have to work out the various necessary blend weights for each area, as well as necessary clearance distances for vertices for different layers of each body part of the character.
Then you have to make sure all the art actually adheres to these rules.
If you’re doing a lot of this (swappable parts and so on) then investing in building tools that verify the correctness of the weights and positions is worthwhile.
For hard plates, you might even need to use a special bone that will move it up/out a bit when the arm is raised, just like you’d have to do for a large hard plate if you were to actually build the thing in real steel.
For soft clothing, you may need to apply even more tech. The Roblox blog has a good write-up of a way to do this: Layers of Genius Behind Layered Clothing - Roblox Blog