I've just checked but the current solution isn't perfect.
Files are compatible between apps, but under iOS, you can't save directly a file in iCloud drive. When you tap on Save button then Export button, the edited home is saved in SweetHome3D folder, and you'll have to copy it to iCloud drive afterwards with Files app. Opening a file found in iCloud drive is more simple: tap on Open button then Import button and select the file you want.