If you visit https://www.photopea.com/ and click the red "Account" button in the menu bar, you can see some stuff about that.

Currently when a user or team "goes premium", ads are removed. That's it.

There's also a "Distributor Account", which allows you to use Photopea in your own site in an iframe without Photopea branding or ads: https://www.photopea.com/api/accounts#teams