Funny story: just last week I was booking a complicated multi-city trip with 5 legs, multiple countries, etc. I tried several times via united.com, and it wasn't showing me any flights available in one of the legs (which I knew they had a codeshare agreement).
I went back to Google Flights, chose all my flights, selected "Book with United", and was able to finish the reservation on united.com (!), with all the flights properly populated, including codeshares.
It's remarkable that the integration between Google Flights and the airlines is better than even the airlines' websites themselves.
But validating and pricing any single itinerary is a trivially easy problem ("P"), so United can handle that no problem. It can't come up with the itinerary, but given a particular one, it will sell it to you.
So no, there's no special magic here.