How accurate is the location of a device in Find My?
How accurate is the located device or person to the position in the map?
It depends upon what is being used to determine its location. The chances of locating a device depend upon which features you selected in:
Set up Find My on your iPhone, Mac, and other devices - Set up Find My on your iPhone, iPad, iPod touch, or Mac - Apple Support
GPS signals can vary. If I am at home and I am using GPS, it usually has a circle around where my phone is located but I can't tell if it is in the house or in the garage. GPS signals can also be very poor and then you're lucky if it can tell within a mile or two.