Fred Trump (Donald's father) built a lot of solid post-war apartment buildings in New York City. The Trump Organization still owns them. Before that he built military barracks for the government. That's the basis for the family wealth. Being a landlord is stupidly easy, you can hire people to do the day-to-day rent collection and maintenance. When it came time to pass the fortune to his kids, Fred arranged to have his children own a "maintenance company" that vastly overcharged for their work, to the point of filing fake invoices. The now "money losing" buildings were then sold to the kids at a low price, avoiding capital gains and inheritance taxes.
Fred's net worth was estimated at $300 million. When you account for the rise in NYC real estate values, that would be several billions now, plus profits over the last several decades. All of Donnie's other failed business projects were sustained because they had the base of profitable rental property, plus some reasonable office buildings added later (Trump Tower, 40 Wall Street, etc.)