Originally Posted by YYC009

I am going to take a wild guess you arrived on AC321. This flight usually arrives at gates C70/C71/C72/C73 as the aircraft usually turns to AC578 to LAX. Once you exit the walkway, you are in the C70-73 domestic gate lounge which is used for the domestic four swing gates.

The different levels separate the domestic, international, and transborder passengers, as Gates 70/71/72/73 are the only four gates that can handle all three types of sectors. The 'basement' corridor you reference is what we call the 'low road' - the corridor above is the 'high road'. Depending on what is happening at the other gates the CAA assigns passengers to either board/disembark using the low or high road to ensure no comingling with other sectored passengers.