GPS units are bike agnostic. In plain English that means you just get additional handle bar mounts for different bikes and use the device on whatever bike. I have mounts on 6 different bikes.

The device figures out speed and distance from the GPS plot you start at the beginning of a ride. It knows the time and moving time so knows avg., etc...,. A Garmin uses a smartphone app called Garmin Connect Mobile to pair via BlueTooth to the device. At the completion of a ride, you press Save and the ride data goes to the cloud, in this case Garmin Connect, which is an online activity tracker. Or you can have that ride data saved to Strava or RideWithGPS.

It’s not like the days of a cycling computer that used a magnetic wheel sensor to determine speed and that was somewhat specific to a particular bike due to wheel circumstance settings. A GPS could be mounted on the family dog and it’ll plot a ride.

For touring you might want some navigation functions such as turn-by-turn directions. For that you can plot a route on Connect, Strava or RWGPS and have a route sent to the device. You might also want to be able to view the map to select roads or alternatives and for that, the larger the screen the better. A decent model designed for touring is the Edge Explore, sells for around $250. Doesn’t have a lot of the functions related to performance functions - power meter, timed intervals stuff, etc... it’s a more basic unit with a touch screen (useful for panning and zooming a map) as well as a
larger screen than the 520 Plus, which is a button control unit.