You can alter NPC weights in-game, but it creates issues with gaps at the neck, hands and feet. To edit NPCs, you need to go into the CK itself and change them there. One NPC will have 25 weight, another might have 66, and so on. Then the NPC will have a number related to somewhere along that slider. It uses a singular body frame, that has a weight slider of 0-100 (thin-to-fat, for instance). This is mainly due to the way that Skyrim generates bodies. The only way you can make NPCs 'truly' unique, is to go into the Creation Kit and fully-customise them individually in collaboration with BodySlide and OutfitStudio. Hence why some Follower mod will have requirements for CBBE/UNP or other presets people have made.
Some have unique bodies (can't recall which, to be honest) but again, it's usually from a preset.
WIth regards to bodies for unique NPCs, it's typically only the faces that are unique and they use a body preset, which affects all NPCs globally.
