They also make a synthetic paper that is stronger and better than roofing felt,,tar paper for the golden oldies like me....

The major problem that we have is that our landlord also operates an appliance business in their back yard (which is super noisy and constantly smelling of chemicals and spray paint), and when we've asked for repairs, they simply send in their appliance guys who half-*** the repair job, and a month later it's broken again anyway.

Is it bleach everything and replace floors, is it some kind of chemical cleaning product like SCOE 10x on the floors and walls, ... 

A lot of times being a landlord it is about minimizing your exposure to liability.If a tenant or their guests or kids were to get hurt on the property, you can easily expect yourself in today's society to be involved in an very costly and possibly very long lawsuit.We advise owners not to leave on lawnmowers, ladders, weedeater's, yard equipment, or even cleaning supplies and chemicals.

When looking at investment property, pet odors smell like money.I bought one of these a few years back and was prepared for chemical warfare. 

Now when you walk it there is a very strong chemical smell and we can’t pinpoint what it is.

I bought synthetic mattress covers that are supposed to take care of this, however I did see this product on shark tank:https://buggybeds.com I've never had any "bug" related issues in 3 years.Mike

If it's the flooring I'm thinking of (the synthetic tiles that click together that are completely waterproof), you might not even have to pull them up.I've had water get under those in my own house from a plumbing malfunction, to where the whole floor was floating.

Electricity, chemicals, scaffolding, spinning blades, etc. 

Cleaning the air handler coils when dirty, changing filter, using A\C pan tabs, blowing out the drain line with shop vac and using chemicals to deter growth in drain lines all seem to help.