I would like to know how to backup my phone data on iTunes and how to merge data from two old phones onto a new phone

Hi, I recently purchased an iPhone SE and transferred the data from my old iPhone 4S using iTunes. My girlfriend is about to do the same, using the same computer, however she has two old phones and she d like the data merged onto her new iPhone SE.
My questions are;
How do I backup my data on iTunes? (I would like to put this on an external hard drive).
How do I save my girlfriends data on iTunes without affecting mine? Do I login in to windows with a different user account?
Finally, how do we merge the data from two old phones on to the new SE?

I don't think you can merge iOS backups. Its one or the other. The closest you could possibly get is to restore from a backup & sign into online synced account to get that data there & then backup that data.

As far as backing up on iTunes, you'll see the option when you go to the iPhone section once connected to the computer. You also don't dictate where the backup goes. It is stored in a special folder on the computer (if you store it there in the 1st place) or in iCloud (Apple's servers). ITunes is able to discern different iPhones so that wouldn't be an issue. You'd see it in the Devices section.

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