The same way every normal app saves documents. It's built into the API for saving. When I close Affinity Photo, or Procreate, or Notes Plus, or any iPad app other than Paintstorm, the APIs in IOS decide how to put everything on hold and then reopen exactly as you left it. In some cases you reopen Affinity or Procreate you're in a gallery which displays all your images EXACTLY as they were when you left the app.support wrote:hm... if you force closing Paintstorm, hot it should save the document? We'll think about it...
What about autosaves, we'll plan to change it a little, like every file will be saved as myfilenam1,myfilename2... myotherfile1... another picture1... but in this case, your disk space will be overload very soon and maybe we'll add an option to control maximum autosave space before files will be rewritten.
Perhaps you need to learn those APIs and do exactly what the iPad is designed to allow apps to do, take care of not losing your documents when you leave the app.