How does the Fortnite Live Event actually do programming or how?

And how did they do it with travis scott and how did they do it with marshmello?

Epic Games / Fortnite of course program such events, there would be no other way, they have to "create" it. The Travis Scoot and Marshmello are cooperations.

Ok, did they actually request these 2 stars or how do they do it?

It's hard to say, it may have been that she asked Epic, but it could be the other way around