They don’t get paid.
I suspect that when it comes to pop culture (sports, tv, movies, music) very few of the stars are known by a majority of people. My guess is that at one point or another, I have heard songs from everyone who sung at the halftime show. But I don’t associate any of the songs with them. I hear a lot of songs that I do not know the artist (more common for more recent music). And I often don’t know the title of the song so if someone says Lose Yourself (even if I have heard the song before) doesn’t mean anything to me. And adding another layer, I would need to recognize the artists when I see them. I have music on my phone sung by artists I couldn’t identify if you gave me a lot of money. Have never seen them and if I have, I never cared enough to put a face to the name (after all, I like them because I like the sound of their voice/music).
And though they are not paid directly by the NFL to appear, the performers understand (and presumably expect) the exposure will benefit them financially. Article above notes that downloads of music of performers in the 2021 Super Bowl increased significantly after their halftime performances. And interestingly enough in terms of the “can’t believe how many people don’t know the performers” discussion, presumably many/most of those downloads were by people who didn’t know the given performers or the particular songs being sung or know they were sung by the given artists (and those downloads presumably lead to people downloading songs of those artists not sung during the halftime show).
I thought maybe I had heard that before! Which was why I put potential pay in " ( )" !!!
I mean, you could argue that they do get paid in the end-the exposure usually leads to more streaming or even actual purchases of their work, new gigs, etc.
ETA: Point made in the post after mine.
If you liked Eminem’s song, you should check out his song “Not Afraid”. I think you’d like that one too. It’s one of my favorite songs of all time.