Who knows what Midjourney uses. We've got only claims in discords to go by.
My guess is they do internally a slightly more careful and less porn/anime oriented version of what the 4chan/protogen people do. Make lots of fine tuned checkpoints, merge them, fine tune on a selection of outputs from that, merge more, throw away most of it, try again etc. Maybe there are other models in the mix, but I wouldn't bet on it.
