Basically he is doing arbitrage.
He buys from people who don’t optimize their house or are lazy and they are leaving money on the table. He will do simple conversions to add rooms like turning closets into rooms. He also deals with difficult people. He is able to make more cash flow because he is doing this in states where there are not as many tenant laws. This is much harder in places like California. So if you want to optimize cash flow on rental properties, don’t do it in California.