I don't think he's a billionaire.
We know what actual billionaires have done in the last few decades. Soros, Bloomberg, Thiel, Gates, Koch brothers, Schwab, Uihlein, just off the top of my head.
Hell, even Musk dropped $44B on Twitter just to more freely troll and ****post about politics.
Those are real billionaires. They spend 10s or 100s of millions of dollar of their own cash on political causes. Their spending is conspicuous and enormous.
Trump has to divert campaign and "Stop the Steal" donations to pay his lawyers. Or not pay them, if he's in a stiffing mood. He had to embezzle campaign funds to pay hush money to a prostitute. He hasn't spent a cent of his own money on anything from the first day he ran for office. Maybe it's because he's cheap and shrewdly hoarding money. Or maybe he doesn't actually have any money that's not part of a shell game of debt pyramids.
It wouldn't surprise me if his kids' profits from his presidency are the only real profits that family has seen since he got paid for the Apprentice.