iPad, iphone linking and usage If I had an iPhone only on my iTunes account, could I use someone else's iPad to download an app to my iphone? If I used the app on their iPad, would it show that my iPhone downloaded and used the app?
Any app belongs to the Apple ID that purchased it.
You cannot transfer it to another account. 

You could download it to another device only by logging in with the Apple ID that made the original purchase.

You do this by going to Settings ➔ iTunes &amp; App Store then tap on Apple ID, then tap Sign Out (of whoever is signed in) Then sign in with your Apple ID.

Everything you've bought with that Apple ID (music, movies, apps) can be downloaded to that device.

If you used someone else's device to purchase an app using your own credentials, it would still be available for later download to your phone - though it would, of course, still be on the other person's device unless you deleted it afterwards. You can't remotely delete content from a device.

You can install apps on up to five iOS devices concurrently.

It goes without saying that letting someone else know your Apple ID login details could be potentially very unsafe.