Does Burger King subsidize their commercials by running Samsung Ads within them?

Cross-Promotion definitely exists. In the US, a lot of iPhone ads are paid for by the carriers so they can put a blurb at the end.

Burger King is actually a weird example for you to use. They use cross-promotion more than almost any other company.

BK was also a leader in cross-promotion. In 1977, they ran commercials using Star Wars advertising while selling glasses with the characters from the film.

Movie studios have been using paid promotions for products since the 90s. Iirc the very first paid movie tie-in was in ET. The studio had planned on using M&Ms in the film but were rejected by Mars. Hershey’s heard about this and paid them to instead use Reese’s Pieces in the movie.