Title says it, if I turn on my VPN while find my iPhone is on, what will happen? How does it work?
I’m fairly sure that it will use the location services (GPS, etc) which is not influenced by your VPN.
Your finder works based on GPS, not IP.
So no worries or problem
My VPN fake the GPS location with the VPN server location, so probably find my device its going to be useless for me
On iPhone? IIRC Apple does not allow GPS spoofing.