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

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 

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?

Simple- exellent relationships with reliable and honest contractors.

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.

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

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

Fyi, the discussion went like this, she said I want a wrap around porch, grey siding with white trim, bigger windows, french doors, stainless appliances, black barn doors and shutters, etc., after which I put my foot down hard and said “dark grey or light grey siding?”

That is a very simple number.