Trump was funneling campaign contributions through his businesses.
Campaign contributions are a norm for Presidential elections, and Trump spent plenty of money from donors for his own businesses. He’s even made it impossible to cancel monthly donations.
This isn’t illegal, but it is novel, which makes it hard to judge just how much money made it back to Trump. Legally he could only pay fair market value for whatever he bought with donations – this prevents insurance fraud – but outside of that, he could spend the money on all personal expenses.
And since Trump has refused to release his tax returns, we’ll probably never know the details.