They just tell the bankers "I'm a YC Founder" and the vault doors open, money spills out.
Probably by applying for a mortgage like everyone else. Outside the clique of so-called Founders the title means next to nothing. How does any business owner get a mortgage? Same as everyone else: down payment and good credit.
It's exceptionally hard for Entrepreneurs to get mortgages with perfect credit and above average down payments.
Maybe now, harder for anyone to get a mortgage. The last time I got a mortgage while self-employed I had to document my income. Not exceptionally difficult, just tax returns and bank statements rather than pay stubs.