I assume the structure could be different depending on which operating system you are using, but within the Field Maps folder on the device you will find folders that are named based on the Portal/AGOL and username. Within these folders, you should see a "mapareas" folder, and then within these folders there should be folders that have the itemid of the maps you are taking offline. Inside the itemid/map folders, you will find your replica folder which contains a folder (named p13 in my current install/version) that has all of the mobile geodatabases. It is probably easiest to just find the replica folder you need to back up (check the date created/edited), and copy that one elsewhere. In an emergency where something happens to the field device, these mobile geodatabses can be dragged into ArcGIS Pro and synced.
How To: Access Offline Edits from ArcGIS Field Maps Directly from an iOS or Android Device (esri.com...
I hope this helps.
Mitchell Grafstein, Horticultural Inspector 1, NYS Dept. of Agriculture and Markets