Originally Posted by eigenvector

1. International to international connections at YYZ will require a cursory document check by immigration officials. The queue is separate from the main immigration area and you are not cleared to enter the country afterward. There is no need to clear immigration and customs unless you need to go landside.

2. Yes, mobile boarding passes are generally available through Apple Wallet with the AC app, or email/SMS. So there shouldn't normally be any need for a connecting passenger on two tickets to check-in at the airport. However, there can be things that will prevent an online check-in, such as being selected for random security screening.