All I know is that he worked his butt off, took good care of his family and is very wealthy.
Real estate in particular is tricky - it is not like making widgets where you can scale to meet demand, close-out excess inventory, drive efficiencies through volume sales and purchases and production output. The up/down cycles in the RE business extend out for years... years of value appreciation, and then years of depreciation. When building from scratch or renovating a property, there are great expenses and a lot of time; and if the timing goes against you by the time the project is finished you may not be able to realize a profit for years, if at all. If you're lucky you can generate enough cash flow to make interest payments.
Here is one great example. Around 2006 Deutch Bank put up nearly $800 million in loans to finance a condominium project on the Las Vegas Strip. The towers went up just in time for the 2008 Real Estate crash. Prices plummeted and there was no way the developer would be able to sell the condos and make a profit on the development. The developer defaulted, Deutch Bank took possession of the property and spent another $4 billion to turn it from condos into a hotel/casino that became The Cosmopolitan Las Vegas.
DB eventually sold the hotel for $1.7 billion - a total $3.1 billion loss for one single property. Trump is not as wealthy as Warren Buffett, and doesn't have the same access to capital as an international investment bank. As wealthy as he is, one bad deal like that would give a developer the size of Trump net operating loss carryovers for many, many years. He could make $300 million a year in profit for the next decade and never pay a dime in taxes because of that one loss. And we already know by the portion of his tax return that was illegally leaked to the press, he had at least one sour deal that did in fact give him something like $900 million in loss carryovers.