Syncing ‘just happens’.
From the user’s perspective you choose a location (Dropbox, iCloud, OneDrive etc) and then a folder and then open/edit/close a map in there - the system then takes care for making sure those changes are uploaded to the cloud and thence distributed to all your other devices. This all happens via a ‘plugin architecture’ provided by iOS. The following diagram gives you an idea of all the pieces.
NB1: Most cloud providers have a files plugin - but for those that don’t (eg WebDAV) there is an app that can help - see FileBrowser.
NB2: The GDrive plugin from Google doesn’t work but the FileBrowser app above will plug into Google and it does work.
The old days…
In previous versions, iThoughts actually did the syncing. It connected to Dropbox servers and uploaded/downloaded as appropriate. This was exposed to the user as ‘linked folders’. These are still supported - but unless you already had linked folders configured then the option is now hidden.
If you currently use linked folders and want to migrate over to the ‘new way’ (using the Files approach) then you should do the following:
Long press on the linked folder (found in the iThoughts folder in the On My iPad location
Choose Move and then choose where to move it too (iCloud/Dropbox etc.)
After having verified that a copy of the folder has been created in the new location:
Close any maps and tap on the Gear toolbar button
Choose Cloud -> Linked Folders -> <linked folder name>
Tap on ‘Unlink Folder’
Long press on the linked folder (again) and choose Delete.