1) If you're flying with Air Canada, you transfer in the same terminal (Terminal 1). Now all flights from Toronto to US have moved to Terminal 1.
2) No need to worry about your checked baggages for outbound flights (leaving Canada). For inbound flights, if your transfer is made in Toronto, you'll need to do clearance in Toronto. In this case, you'll need to get your bags. In short, the first transfer point or destination in Canada will be where you'll do your clearance.