my iphone 11 has a broken unusable screen how do i unlock to back up

If you cannot access the screen to unlock it, then it cannot be manually backed up at this point, it's too late.

If the option for iCloud Backups was turned on, you can connect it to power and Wi-fi and hope it backs up eventually. "iCloud automatically backs up your iPhone daily when iPhone is connected to power, locked, and connected to Wi-Fi." Back up iPhone - Apple Support

Outside of that, backups are preemptive measures. You are supposed to backup while you have access to the device so when something happens and you can't access it you have already a backup you can use.

You don’t. You need to back it up before something goes catastrophically wrong, not after.