There are two ways to loop in the Desktop app. Using the Classic view and In Tab Sync View.
Tab Sync View
When using Tab Sync, you actually select the section from the tab to loop.
Loop sections easily by clicking and dragging across the notation. It snaps to the nearest note, rest or barline, for ease of making “perfect” loops.
In the Classic view, use the loop icons shown here.
Start Loop / Stop Loop / Play Looped Section