For questions, bug reports or requests, write to yugo1997zazen@gmail.com. It helps to include your device, its iOS version and what happened.
Yes, when both are signed in to iCloud with the same Apple ID. Check the state in Settings → iCloud Sync.
On a device with Apple Intelligence it works on the device as it is. On other devices, download a model in Settings → On-Device AI. Without either, you can still make a board by placing your photos into a sample design.
Add a frame from a board's “…” menu, then follow the steps in Settings → Widget or Wallpaper.
Deleting the app removes the boards on the device. iCloud data can be deleted in Settings → [your name] → iCloud → Manage Account Storage.