YYZ and YVR US -> Canada
 

I'm not 100% certain what the question is, but if you arrive at YVR from the US on Air Canada or WestJet, you can connect to a domestic destination without going through customs or claiming your bag. Keep eyes peeled for staff handing out cards to identify you as an ITD (Intl -->Domestic) passenger. As you descend the escalator/stairs into the main immigration hall, make a hard right and clear CBSA immigration at a kiosk* in a separate ITD area (almost underneath those same stairs). From there, you'll be herded into a glass corridor and onto a bus waiting outside that will drive you to Gate 29 (post-security) in the C pier, from which you can stroll to your domestic departure gate. No additional personal screening required. Bags will be sent to your domestic plane and waiting for you at your destination.

*the ITD-specific kiosk area doesn't have dedicated Nexus kiosks - only the regular PIKs used by everybody. But passenger volumes should be low enough that you wouldn't gain any wait-time benefit from a dedicated Nexus line anyway.

That seems strange, you are supposed to clear customs at your first point of entry to Canada, since your ultimate destination will not route you through customs, as you will be a domestic arrival.

You DO clear customs; just at a slightly different location. You input the relevant info into the automated kiosk, and it spits out a receipt. You present this, along with your passport to a Border Services Officer, who will look at your documents, and possibly ask you a few questions before okaying you to go through the doors that lead you to the awaiting bus.

I did do this, interestingly the YVR staff told me not to bother with the eDeclaration - apparently it takes forever to parse the QR code and locks the machines.