Tap/hold on a measure until it selects, then slide your finger (down, across etc.) for the selection or drag the blue selection handles to set the selection.

Tap the arrows in the first selected measure or the Play button to begin loop play.

 

 

Chromebook (ext. keyboard, mouse)
Click/hold on a space until it selects, then drag down, across (etc.) to complete the selection; or after the space selects, drag the blue selection handles to set your loop.

Play
either click the arrows in the first selected measure;
or click the Play button;
or press Spacebar

 
 
 
Older versions (v2024.6 or below)
Tap (click) and hold the first measure of the section you'd like to loop, then tap (click) and hold the closing measure. The loop measures will select.
Tap/click anywhere in the selected area for Play.