How can I get permanently deleted photos back

I'm not sure what you mean by that. If you back up your device regularly (https://support.apple.com/en-us/HT203977), you can restore from that backup provided the most recent one was made before you deleted photos. (https://support.apple.com/en-gb/HT204184).

Otherwise, your photos have been erased.

You cannot. Permanently means permanently. You can restore photos from a backup made to a computer if you have one.