I only know what Midjourney says on their page.
That you can do what ever the fuck you want with the images created from your text. Sell it, burn, wank to it.
And that Midjourney reserves the right to do what ever the fuck they want with the images created from your text. They do highlight however that each time you create content with their software it will be shared with everyone else publicly and that everyone else can use it for their purpose as well.
- Exception when you're a business/company making more than 1 Million dollar per year or something. Then Midjourney wants a part of your cake.
The broader issue is the legality of it all. Right now, Ai generated content is a really really large gray area when it comes to copyrights. No one really knows. It's the wild west for now. And it will remain so till someone challanges it in court I guess and you get presedences and all that.