I use the keyword wallpaper in addition to background.
Customers can select 9:16 with filter options, so it is easy to find for them.
iphone and android should only be used for editorial content that actually shows an iphone.
Our buyers are design professionals, I doubt the normal iphone users would open a subscription account with Adobe to buy some backgrounds.
They get them from other stores.
But mobile format is also used a lot for online advertising, so I am trying to create more content for it as well.