How to transfer files and photos from 2 iPhones using same Apple ID

Be very careful. There are two separate things to consider:

1. Transitioning to a new iPhone from a previous one - see especially the Quick Start information here: Set up your iPhone or iPad - Apple Support
2. iCloud: You said “I have some backup on iCloud storage”. Hopefully you do not mean an iCloud backup; what you mean is you house some data in iCloud. That data is associated with your Apple Account, and will appear on your iPhone once you sign in using your Apple Account.

If you are using iCloud Photos on both phones, with "Optimize iPhone Storage" selected on both phones, and iCloud syncing and backups are paused for lack of iCloud space, then upgrading the iCloud+ plan should help.

When you are out of iCloud space, backups fail, and synchronization stops. This usually translates into having data that exists only in one place: on a particular phone. Anything happens to that phone (theft, forgetting the passcode and having to reset the phone, phone is run over by a truck), and you lose all of that data.

It may take a while after you upgrade the plan for the phones to finish synchronizing to iCloud.

Hopefully,

• You have been using iCloud Photos to synchronize photos and videos between your iPhone 8 and iPhone 12
• They both have had a chance to upload all of their photos and videos to iCloud. (E.g., you have enough iCloud storage space that syncing did not stop because of lack of space.)

If each phone has its own set of photos that is not synchronized to iCloud Photos, but that is included in backups, things are going to be more involved if you want to preserve all photos and videos from both phones.

If your phones have photos and videos that are neither synchronized to iCloud Photos, nor backed up, that is the worst case.

My understanding is that if you restore from a backup, it will wipe whatever is on the phone to which you restore the backup. You won't be able to merge the data on the iPhone 8 and iPhone 12 onto the iPhone 15 in that way.