I strongly doubt there are any GANs involved, they are rarely used in production entertainment, and a solution doesn't really exist for vector geometry afaik. Likely very few scans as well. Just lots and lots of artist hours.
As far as scans go, it's much cheaper than you would imagine to get high quality face scans. Rigging them for animation is the real challenge, and I'd be really impressed if they were using GANs for rigging.
For state of the art in this space.. check this out: https://twitter.com/ak92501/status/1311832183078879232
I can imagine they might be using scans that they fit their model to to generate some blendshapes, but it's just as likely that they don't.
Just look at the rig controls, it's exactly a better, and more polished version of rigging controls that have been used in animation studios for years. I think they are just doing what everyone else is doing, but with just a ton more money.