The other two that are commonly used is (1) 100 hours and more than anyone's time in the activity or (2) substantially all the time in the activity. 

The most common answer, especially in recent times, is highly levered, floating rate debt.This is not the only answer, by any means. 

Unless a different one would be better that you could suggest   If you are house hacking you can utilize 3-10% down. 20%+ is great to put down but not always possible.

Basically common contract or agreement law.

This is a common 'trick" we use in flipping.

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They sell a 4 unit for close to 1 million dollars close to Jacksonville. they want 250k or 300k down. there's no upside. you can google the properties on biggerpockets and there isn't a lot of satisfaction. it's a slow return and get rich slow strategy. our focus is always as a builder to add maximum value, lower cost as low as possible, build and rent and build and sell the same product and raise prices to help investors out. we have a large market share and large purchasing power in our market in columbus ohio. the one strategy I never liked about turnkey new build brokerage companies like them and build2rent or others is they don't really build close to urban centers and most are single family homes. the numbers don't work on single family homes. if single family was the way to go more companies would be doing it. but by far the most common type of new construction built is three story walk up apartments.

., longer-term, amortized, require escrow, commonly modified b/c of prepayments, etc.)? 

Duplexes are very common here and are much easier to find than a Quad as well.

With the purchase, the rehab, the closing costs and the holding costs (hard money, taxes, insurance, utilities) our all in is around $200,000, but we have created $40,000 in equity.