I'm brand new to DaVinci resolve. I've figured out how to key out a green screen and how to use the motion tracker. Is there a way to attach the keyed footage to the tracked footage? Or should I just do this in After Effects?
It depends on the nature of the scene and the composite. I think you can do simple composites within Resolve, but for really precise results, you're better off using a dedicated compositing program like Fusion or After Affects or Nuke or something like that.
Pages 970-972 of the Resolve 12.5 manual go into detail on how to do a match move to composite a sky replacement and match it to the foreground movement.