Edit: I realized that I made a mistake after reading replies below. I actually gonna arrive at T1 and leave from the same terminal.
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I will depart US and transfer through Toronto YYZ to China with Air Canada, arriving at T1 and leave from T1. There is a long overlay time in YYZ, more than 7 hours so I gonna have to stay in some lounges.

Question:
1, Is this considered as transborder or international?
2, I read that the Plazza premium lounge has a 3 hour limit, as I priority pass select holder, should I get out and go to a different one? I know there are a few lounges in YYZ. My PPS comes from Citi Prestige card, is there any limitation on how many lounges I can user per day/airport?

My return trip is even worse, arriving at YYZ at 4pm, and won’t leave until the next morning at 9am. I will have to get out of the airport and find a hotel. Do I need to go through Canada/US custom when arriving? If so, will I have to go through custom again when arriving in the US?

TIA