My original SFR was 1100sf and I added 2 units: an 800sf SB9 unit and a 750sf ADU (converted garage plus addition).

Or, in Providence, RI they took the oldest indoor "mall" (Arcade) and converted the second and third floors to apartments.  

Duplex Development Made Easy - SB 9 permits property owners to convert single-family lots into duplexes.

What is the realty of converting a new Warehouse build into apartment spaces?

You also tend to get more frequent and better feedback than with an STR, if you treat your residents well.The last quadrant, unnecessary but not obviously so, is usually as a result of either converting an STR to an MTR or from remodeling above the standard needed.

All the rest can be subbed out unless you are doing a full remodel by adding square footage to the house/converting a space (bath/bed etc).

Actually you'd never hit that because eventually the debt pushes you into a margin call and forced sell and you'd be at or near $0 in just handful of years at best. 

You are just throwing money away and will eventually fall victim to squatters.

Yes, most lenders will require you to live in a primary residence before moving out, refinancing, or converting into a rental.

Not like the hype on Youtube which will eventually get you into trouble.