So the tricky thing with iPhone 7 series is they are VERY boot loop prone. This could be any number of things, it can be difficult to narrow down. This could be an Audio IC issue, or a baseband issue, or a NAND issue.
Usually a sensor issue will allow it to boot all the way and will reboot after some time. I am more likely to think it is something else.
Is this something you need to get data off of? Or the person just wants it repaired. I might see if it will complete an update and make note of what error it gets if it fails.