Backup 8 phone when phone is dead

Depends on how dead it is. There are some forms of "dead" such as a malfunctioning touchscreen or dead display where everything inside is still fully functional. It's possible for such damage to be repaired by just replacing the front screen assembly and you can get all the data. Or where it's possible to do a backup, turn on alarms, check the battery charge level, etc. However, that requires that it has been "trusted" with a computer. There might also be automatic backup of some of the data to iCloud. You can check that by logging into iCloud.com with your Apple ID and password.

View and manage iCloud device backups - Apple Support

What are you looking to get out of it. A lot of dead iPhones "serviced" by Apple use end up getting replaced. If that's OK with you, then Apple is a good way to do that because getting a refurbished iPhone (in good working condition with a new OEM battery) is pretty good. It may not be so good if your primary concern is getting the data back. Apple makes no promises about the data and makes anyone receiving service acknowledge that the data may be erased or otherwise non-recoverable.

You cannot. You backup devices before things like this happen.