How do you get them to loan you the money if you don’t have to pay any interest?
They figure out what the “interest” would be, add it to the agreed upon amount, call it a profit, and divy up the payments so that you pay that total.
Then they say, see, we did not charge interest, just our profit for the transaction.
Which leads to the interesting question of how do you refinance a mortgage. You have to pay the full “interest” no matter how long you actually have the loan, so I guess refinancing never happens ? You buy a $150K home on a 30 year mortgage, but owe $400K even if your life changes and you want to sell 6 months later ? That must work out great.