I think most donors are consumer oriented businesses that hope to get a return on their "investment" by having the players promote their products. I don't think the money is coming from places like PPG or US Steel.
I'm sure a fair amount comes from collective sponsored events like lunch with Franklin and autograph sessions with players. Fanatics are willing to pay for that stuff.
I assume it also comes from wealthy individuals who don't have anything better to do with their money. Over the years people like Phil Knight (Oregon), T. Boone Pickens (OK St), and John Tyson (AK) have given $millions to their universities. It's historically gone towards buildings which are a legacy. I don't know how many want it to go to pay players. But isn't NIL supposed to be in exchange for work? Can it just be a gift to players?