Merging accounts

Hello,

You appear to have signed out of your device with one ID and are signing in with a different one. If you do that and do not erase the phone between steps then data and apps from the previous account are still on the phone. If you the sign into phone with a new ID the data from the old account will be merged into the new ID. This can be messy if you are not doing this intentionally since if, say, you have contacts from the old account and you are using iCloud to sync contacts in the second account, they will then merge and get synced to the second account's iCloud. As I said, sometimes this is done intentionally as an attempt to merge two IDs, but if you do not want this to happen then it can make a bit of a mess and you need to ensure you set up the device properly for the use of a different account on it. This document describes how to do that: What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support

Merging accounts basically means, if you signed out of your Apple ID, and have a copy of your data saved. And sign in to that account again, and you'll have the option to merge both data from both copies.