How can i restore my photos from dead iphone 8 to iphone 12

if the phone is dead, you will have to have a backup that contains the photos.. then restore the new phone with that backup.

OR have them stored in iCloud.

Locate backups of your iPhone, iPad, and iPod touch - Apple Support

Restore your iPhone, iPad, or iPod touch from a backup - Apple Support

If you don't have a backup and don't have them stored in iCloud, then you will not be able to retrieve them from the dead phone. (unless maybe the phone could be repaired in order to get those photos)

If you back-up your iPhone data to iCloud.. simply you can connect your Apple ID to another device and download the photos that you want to recover (Sign in via browser)..

link:

https://www.icloud.com/

Or you can do this using third party software to recover your photos..