Mostly, advertising. Radio, TV, Internet, as well as the agencies that create the ads. Americans are utterly blanketed with political advertising during our interminable campaigns.

He's extremely limited in how much he can give directly to the campaign, but as long as you've got a fig leaf that you're not actually working for the candidate, you can spend as much as you want.

That's just the First Amendment in action. The Citizens United decision extended that businesses, as individuals pooling their money, but when you're a trillionaire you don't even need the Supreme Court's approval.