They would've put at least 150k down wouldn't they? This also assumes they just moved in. If you have a 1.5 million dollar home in SF today you likely bought it for less than that at some point in the past. These people have net worths that would allow them to buy a nice house, almost anywhere else on the planet for cash.
You're making a lot of assumptions here, none of which might be true, and which are incredibly individualized.
"I have a mortgage on a house" tells you essentially nothing about a person's finances, how they financed the house, their assets, their monthly expenses or their debt-to-income ratio.
Some people might have put 20% down, avoided PMI, have no other debt and only put down 20% because they wanted to hold onto cash. They had a good credit score, and got a reasonable interest rate.
Others may have utilized any one of a number of loan programs to put down > 5%, which they financed through loans from family, or emptying their 401K, got a not great rate, have PMI, have student loans on top of that, a healthcare debt, etc.
You have zero information beyond "well, they have a house."
