Restoring different iPhone version and factory reset

Hi folks, hoping you can help me with this one. It probably should be simple but I’m struggling to get my head around it and don’t want to accidentally erase everything... 🤣

So I was running two copies of the same iphone, having got a new device, migrated the content across and continued using the previous one along side it. Old one was 6, new is 11.

Old phone subsequently turned into an expensive brick (having, I hope(!), backed itself up first).

Now I want to use the most recent backup of the old (deceased) phone on the new one.

Do I just erase everything from the new phone and hope I can install the backup from the old one? Is there a better way of doing this?

Also the new phone was second hand and is glitchy - regularly cutting out on landline calls to multiple numbers, buttons on screen don’t work on sites that really should be compatible (Facebook and Vodafone apps, both updated to latest version), and today lost the screen unlock bar at the bottom of the phone so wouldn’t swipe up to let me into the phone.

So I want to do a factory reset in the hope that will help. Basically I need to know what order to do things in!

Thanks in advance for your help!