How does this work, people working in the stores in the USA terminal of, say, Vancouver airport, before starting their shift go through USA immigration and customs every single day?
They go through the employees entrance.
In almost every airport, there are special entrances for employees to access different "sensitive" parts of airports, either parts that are not allowed for passengers at all, eg. ramp, or parts that are allowed for travelling passengers only, eg. duty free and gates. They go through regular security check (sometimes it is even more what passengers go through), sometimes there are security checks when they are leaving as well especially for duty free people, since they are not allowed to get any items from the duty free out of the duty free zone.
Regarding immigrations and customs, No, they do not go through that.
Note: I have used these employee entrances myself as I am a cabin crew member in many airports, including some US airports.