A) Loans need to be paid back, with interest. The person must either be selling assets or drawing in other (taxed) income to pay back the loan. A loan could delays the taxes to a future year to let someone buy a house or yacht or whatever without the full tax burden in year 1, but they still ultimately pay all the taxes
B) If they die while still having outstanding loans, their heirs pay a 40% inheritance tax on everything above like 10 million, so there is no magic avoidance of taxes there, just a change in whether it's capital gains tax today or inheritance tax tomorrow.
I'd love to be disproven if someone can explain a real tax loophole, but as far as I can tell, the "Billionaires avoid taxes by taking out loans" thing is completely untrue.
I'm not an expert on this, and I could be misunderstanding some subtlety here.
[1] https://www.irs.gov/statistics/soi-tax-stats-estate-tax-fili...