I have 8 SF properties today and would not have been able to do so if I was contributing heavily in my 401k.

Depending upon whether they are a bank or a trust company, custodians are heavily regulated by the IRS, DOL, and possibly the FDIC, State Banking Commissioner, and the Comptroller of the Currency.

It's almost all interest for several years as @Erik Whiting mentioned above.Investors are BRRRRing with very small margins from heavily leveraged positions.

Because I own my own business and deduct HEAVILY off it, I qualify for zero mortgages, even with perfect credit.

For the record, if you read their profiles, you will see that all of these billionaires are heavily invested in Los Angeles real estate (see below). 

@Dhaval TaylorI have an STR that relies heavily on bookings from AirBNB.

Any lender who doesn't require some of your money in a project is playing a dangerous game with the odds heavily stacked against them.  

Asbestos and lead paint are two heavily controlled materials and landlords can end up on the hook for remediating these. 

The $50K in extra salary is taxed heavily.

I also rely heavily on Craigslist to get an idea.