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.

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?

Currently most places operate in a grey area or require a simple permit (there are some toxic towns that we'd steer clear of).

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". 

I currently own & operate two STR‘S in SoCal USA, both fee-simple, no leasehold, so I’m new to this concept. 

Many of the tax planning service companies referenced herein want charge $5000-7000 for tax planning services which seems exorbitant for someone like me who is a single W2 earner with a pretty simple tax return.