2 questions!

[Mikey Cas]

Hi guys, I have two questions that I have been wondering about. First is about financing a property, second is about building up properties.
1. I have read that it is possible to pool together investors to come up with a down payment, and then get a mortgage for the remaining balance. Basically making it no money out of pocket for the downpayment. So for example, If I wanted to buy a duplex, live in one side rent out the other, apply for FHA loan with 3.5% down, and I could get investors to come up with the 3.5% or use seller financing to hold the first mortgage. I think FHA allows that. OR is this just a method for commercial real estate deals?
2. Second question, for investors who build substantial portfolios of property, hundreds of rentals, etc. How do the banks keep lending money? I know they look at your income, cash in bank to cover 6 months payments etc. I know in commercial they look at the buildings potential income for loans, but for people who own multiple residential units, how are they able to show the bank they have income to cover millions in mortgages? Myabe I have it all wrong lol.Im just trying to understand how it works