Admittedly I haven't flown United in a couple of years, but my experience with them was the same as with every other airline: when booked on a codeshare (i.e., Airline A issues the ticket, but Airline B operates the flight), the itinerary emailed to me would always say "Check in at the (insert operating airline) ticket counter".

At least on the case of Air Canada and United, that's code-share flights. If you book via the Air Canada website it's very very obvious whether you're taking a real AC plane or not, but third party booking sites tend to hide that.

In case it helps, the United code-share flights are numbered AC[345]\d\d\d.

The information is there, although in different placed depending on which website is used. I just have to remember to check the fine print, at least when I am the person actually booking the ticket. When i'm not, then I have to check the boarding docs for the little "operated by" tag. The whole scheme is not something you expect from a supposedly national airline like AC.

I've also had this happen with Alaska and United.

Are these codesharing flights? I'm usually picky about choosing the exact right flights that are actually operated by the airline of my choice so that I get what I want.