Welcome Waldo.
Several methods showing how you might use different types of geofence are described here>
Location sensor (ringed fence / circular ringed fence) - #2 by SteveJG another method using javascript is described here> Geofence. Check if a point is inside a polygon. JavaScript. Map - #3 by Juan_Antonio
Also, How do I program MyLocation to alert when in proximity of required location? - #5 by SteveJG might be interesting.
How you accomplish this part depends on how you coded the part of the app that sets up your locations. You haven't showed how you did that? Post your Block code please.
One method could be something similar to National Park Tutorial (Markers) updated . It has lots of 'locations' saved and can determine the closest to the user.
The arduino part can be achieved by
if .. then statements and a Clock to constantly poll the user location to use Bluetooth to send the two digit value to your Arduino.
You want a Tag for your Firebase for each Location. Use the FirebaseDB tool to create as many as you need.
There isn't a tutorial showing how to merge all these requirements.