They are usually digitally inserted into the picture. Same as the ads behind the batter at baseball games. There may be ads placed there by the stadium, but the TV people are going to cover them up with ads of their own, unless the same sponsors that are paying for the TV time paid for the stadium ads.

You’ll notice that with hockey games and figure skating contests as well. I actually went to a local figure skating contest, and then saw the video on TV a few days later, and all of the “boards” had ads on them, even though in the rink they did not.

It’s an easy way for advertisers to put local ads on there. Different cities will have different ads.

It was kind of weird to go to the ice rink and see that the boards were just plain old white.

Some stadiums do actually paint the grass with adverts that are proportioned to look right from only one camera position. I’ve seen it done. But mostly these days they’re digitally superimposed.