Its not simple like you'd think. 

The most common in simplest form that has been deemed as most passive is a simple, long-term rental.If you have not purchased a property for yourself, yet, I would highly recommend getting started with house hack, and learning that way.

To get a better idea of what your ideal build would cost, find a build you like on HAR and present that to a builder so they can see the pics (finishes/design etc) and with that particular square footage they can give you an idea of what it will cost.

Designed to prevent the ground from shifting.

Depending on the state or jurisdiction where you reside this is a fairly simple process.  

When I managed properties the owner would regularly call me in his office.... why did we spend $1/sqft to paint this apartment, why did we replace a fridge, what does it cost $2500 to do a simple make-ready on a 1 bedroom luxury apartment?

If your own real estate directly, a simple LLC will be more then enough to get started 

Normally 3-4 bed units are 1000-1400 square feet on average hence if you find a property where the average square footage per unit is 1300 but its listed as 2 beds each, there is a high liklihood you can find additional bedrooms increasing value with some simple carpentry work.

Simple- exellent relationships with reliable and honest contractors.

One simple fix for that would be to do a "partial 1031 exchange".