It's vague. There's conflicting things. Apparently if you have a commercial license to midjourney the copyright is yours but not exclusively? Seems like something copyright law has yet to come to terms with.

In the meantime people probably shouldn't look at the AI art produced by the apps used in this thread but rather at the stuff midjourney can produce. There's a very big reason why some are thrilled and others not so much. There's an artist here in the Galleries that produces AI assisted art. Pretty cool work. Sci Fi Fantasy type work. The AI use is obvious and the artist is very prolific. One can only imagine the volume and quality of the work is commensurate with the reliance on AI. The words Not Humanly Possible come to mind. I'm unnerved by it, I can't help it.

Here's a good article about midjourney with a lot of fantastic images and some interesting commentary...

https://www.creativindie.com/midjourney-ai-for-book-cover-design-how-is-this-legal/