I want to weld this multi-part belt to an NPC, but when I create individual WeldConstraints in each part and weld them to the HumanoidRootPart, nothing happens when I try to animate it.
Use a rig plugin (Like for example Edit Rig Lite) and there you can weld more easily the parts (because the plugin already configures what is needed to weld)
You just need to download the plugin, open it, select the Torso (not root) and the accessory, and then press a “Weld” button that appears, then you just close the plugin and it’s all configured so you can animate. (and be sure all is unanchored)