There is one base figure - Genesis, Genesis 2 Female, Genesis 2 Male, and now Genesis 3 Female - for each line. The geometry and rigging is in that though joint centres can be adjusted for different shapes.

The UV set is a surface property, so they can just be swapped out by material presets or by the user in the Surface pane. The different base characters have, as they did with Genesis and the Genesis 2 figures, their own UVs to avoid any stretching but there's nothing to stop people using a different texture set, with a different UV set, on them - or even using different UVs on different surfaces, though aside from eyes and teeth there would be few occasions that was desirable (perhaps for face and make-up on the head and second skin gloves or monster skin on the hands or legs, with clothes hiding the join). G3F does have a new split into surfaces, meaning that old textures can't be applied via a UV set, however (as with the move from Victoria 3 to Victoria 4) - we are all hoping for something like the old texture Converter to address that.

Clothing can generally be autofitted - at worst from older generations (four is supported out-of-the-box) to Genesis then from Genesis to G2 then from G2 to G3 - though there are clone shapes for G3F (and G2F/M) in the store to allow a more direct conversion. Morphs can be converted with GenX 2, though G3 support is not yet available (I think the author is planning an upgraded version when G3M is out).