Adding bridge to a loop or adding different chords in GarageBand on a Macbook Air

I'm working on a song using guitar and drum loops. The loop only has two chords, C & G. If I wanted to add an A Minor or D or F chord to the progression, how do I do that? Or, if I want to add a bridge where it goes into a completely different chord progression, then back to the original progression. How do I do that. It appears GarageBand works differently on a MacBook Air then on an iPad. I'm on a MacBook Air. I have an iPad as well but haven't figured out if you can create things on the iPad and transfer the file to my MacBook Air. I'm new to GarageBand, so please make it simple.