Pulled from: https://motionarray.com/learn/after-effects/loop-video-animations-after-effects/

Start with a piece of footage in your composition you want to loop. Grab a stock video clip if you need one to try out.

• Identify that piece of footage in your project panel.
• Right-click and select Interpret Footage > Main.
• At the bottom of the window is a Loop option. By default, it’s set to 1. Change this to loop however many times you like.
• Now you’ll see the footage layer is longer, reflecting the number of loops. Extend the handles to the right.
• Now when you use this piece of footage it will loop!
• This method is nice and easy, but it will only work for footage.

Now let’s say you have a precomposed animation that you want to loop, you would have to approach this differently. Here are the steps:

• Right-click on the precomp and select Time > Enable Time Remapping.
• Option + click the stopwatch for Time Remap.
• Using the Add Menu, select Property > loopOutDuration.
• You might find yourself getting some blank frames at the looping point. To fix that, simply go into your precomp and extend the layers past the end of the timeline.