Sometimes the rpa file doesn't get discarded completely but only a part gets re-written. Try to take the rpa file and move the file somewhere else and put the new one in.
Other sites ar not safe compared to itch.io (I don't know which ones are your favourites so it might be cap) so if you are not sure of a site you just need to be patient.
When you download the zip file you need to go to the directory that the game tells you to go to and put the RPA file (that is in the zip) in the directory
It all ends up in Apple being so secure that it doesn't allow apps not installed from the app store to run. Honestly for now there is nothing that you can do, just get an android