See the Related Article, linked above.
Unless the person deliberately shut off Find My, it is enabled.
Members of the same Family Sharing group can check the location, as can the person with the Apple ID can log into Find My by logging into iCloud.com/find
If no location is reported, then the iPhone is without communications, its battery is exhausted, is powered down, damaged, or has been disassembled.
If the iPhone was stolen, there is a good chance the owner will receive messages purportedly from “Apple” or “the police” or some other entity, seeking the Apple ID credentials. Supplying those credentials will end badly.
If it is powered off, or if Find My was disabled, then you are limited to looking for it the old fashioned way; by looking where it was last known to be.