You will need to check in with whichever airline is actually flying. It may be an AC flight number but it might be a codeshare with US. Your itinerary will provide this information. I'm not sure if AC has a dedicated counter in PHL, I have only connected there.

You are correct, you will not need to claim your bags. They will be transferred automatically for you. Follow the signs for international connections, show your passport and head to the gate.