You can add new loops to your timeline by browsing the library and dragging clips to your track timeline.
Click on the "Eye" button in the bottom right corner of the window. This will open the right sidebar that was previously hidden.
In this sidebar, you can access the loop library that contains all of GarageBand's built-in loops.
Click through the instruments to narrow down the loop choices. Selecting one type of instrument will gray out all the other options, but you can reset this by clicking the Reset button.
When you find a loop to use, click and drag it onto the timeline. It will automatically create a track and an audio region for the loop.
You can add multiple loops to the timeline and play them together.