SEIDS breaks down Logic Pro's Apple Loops library, over 33,000 royalty-free loops you can drag straight into a project.
Audio loops go on audio tracks, MIDI loops go on instrument tracks, and both let you click the arrow icons to swap in different variations from the same pack without starting over.
Pattern loops work like a step sequencer. SEIDS uses these mainly for drums, and double-clicking one opens the sequencer so you can tweak the pattern directly.
Session Player loops bring in Logic's AI-driven performers, worth exploring once you're comfortable with the other loop types.