Your bags will be tagged to YYZ, but if you are going from a united carrier to AC carrier, I believe you have to claim bags on the carousel and then resubmit them after clearing customs. If you are on solely united, you will get a "t" tag and they will be transited through. (watch for the "t" tag at PTY) You can use the "carry on" only line at immigration (although that is not always faster). Turn left and then proceed downstairs, go out the customs hall, pass the baggage resubmit and then up to security. I had a TSA "pre check" on my boarding pass so that would speed things up too. Then go through security and head for the train.