All USA Federal income tax (IRS income tax) is based on voluntary agreement with the IRS. When a person fills out an IRS form like IRS Form W-4, he states (though not directly and formally) that he is accepting the IRS as his taxing authority. This is where the IRS gets their authority from... the people who fill out the IRS forms. In Trump's case, he probably signed many IRS forms that indicate that he is accepting the IRS as his taxing authority.
In the case of the average working class person, if the person does NOT sign any forms that say/suggest that the IRS is his taxing authority, he doesn't owe taxes. If he fills out the IRS Form W-4 for/with his employer,
and on every line places "n-a,"
and writes across the form "EXEMPT,"
and signs it "non-assumpsit (no contract),
his signature,"
his employer is required to NOT withhold,
and the IRS is required to not tax him.
The problem is that because most people have volunteered to pay, employers may not listen to the form, and IRS people will often make trouble for people. They simply don't know the law, just like the rest of the people.
That is the start of a letter-writing campaign by the employee, instructing the employer and the IRS to not withhold or tax. Depending on the ferocity of the attack against the employee, the letters might go on for 2 or 3 months, but they could go on for a lot shorter of a time period. Following that, the employee will need to take his employer and/or the IRS agent(s) in question to court seeking damages.
I am not going to go into detail regarding what to write in the letters, or how to take them to court, in this post. You can find the information, if you are serious, here -
https://www.youtube.com/c/CraigLynch/videos.
In the case of Trump, if he is guilty of not paying the appropriate amount of income taxes, it is because he said that he was liable to IRS law, and then didn't file the right forms in the correct way. It is up to his attorneys to battle it out with the IRS. Because the IRS Code, and the Code of Federal Regulations for the IRS Code, are so convoluted (they include dips into Title 27 which is the Bureau of Alcohol, Tobacco, Firearms and Explosives Code, and the regs for Title 27), it could take a lifetime to determine who is right. The IRS might simply drop the case because it would be too expensive for them to fight it.
In our eyes, it might look like Trump didn't pay the taxes he legally owed. Our judgment might be right, or it might be wrong. It might not seem fair to us - because we have unwittingly been duped into voluntarily accepting the IRS as our taxing authority - but if the IRS won't fight Trump, and if Trump is getting away with not having to fight, there isn't really anything that he is doing illegally.