iCloud backup from new iphone is lost when restoring old backup

I bought a new iP7+ and started using it then i back up on iCloud. I also back up my old iphone 6s. To restore, i Back up my 7+ again and reset the phone. When i transferred old backup(6s) to 7+, the recent backup on 7+ is lost. On the iCloud backup in 7+ it says “Never backup” when i just did few minutes ago. How do i combine the backups?

Ps: Just before restoring, there were 3 options which i didnt really understand. I just chose the top one that says “Update the backup(i think)”

Apple has no merge feature in any iOS backup. The restore operation implies a blank slate, tabula rasa, wiped device for iOS and TVOS and iPadOS.

The solution is to store data in a cloud synced manner or otherwise import data but backup is purely for a clean restore.

Worst case you restore a backup and export or sync the data you need so you can wipe and restore and then Merge manually on the more current backup. Then layer on the loud synced data or imported files.