The vast majority of his tax avoidance would be done on the business returns, as would, if it were occurring, probably all tax evasion. Most Trump companies are privately held and the vast majority, I imagine, are formed as partnerships and S-corporations which provide some specific tax advantages to a business owner that I can’t get into in great detail here. Each of these companies should issue, on an annual basis, a form K-1 to all people and entities that own a percentage of them. The K-1 reports several different types of income or loss, all of which are totaled up on Form 1040 Schedule E, and taxes are levied on the balance of income reported. For people that own a lot of businesses, as Trump does, it can get complicated rather quickly.
The complexity of the tax code is a feature, and not a bug. There are people in my profession who are paid tremendous amounts of money to find legal ways for business owners to report the smallest amount of income and therefore pay the least amount in tax possible. Very wealthy people and corporations can have years in which they pay no income tax without being guilty of fraud. It’s possible. I don’t have enough information here to say for certain that Trump’s returns are fraudulent. I wouldn’t stake my status as a professional on such claims, in any case.