According to the Due Process clause of the United States Constitution appears in two places of that document. Both instances are in Amendments to the text itself. The first is in the "Bill of Rights"--the first Ten Amendments that were added within a year of ratification of the Constitution. It's a measure of the importance of the principles enshrined in those first ten amendments that there was enough concern about the failure of the Constitution to expressly affirm these principles that James Madison himself took the leading role in amending the Constitution. And so, in the Fifth Amendment we read:
No person shall ... be deprived of life, liberty, or property, without due process of law.
Likewise, in the wake of the Civil War, protection of civil liberties was regarded as so fundamental for the the union of states that is the United States that it was regarded as necessary to explicitly extend that same principle to the states, in the Fourteenth Amendment:
... nor shall any State deprive any person of life, liberty, or property, without due process of law.
Typically, in a law school course on what's termed "criminal procedure", the idea of due process of law is traced back to the beginning of actions that include some infringement on individual liberty or privacy--a search, seizure of property or person, arrest, etc.
With regard to the Flynn case we often hear the complaint that Flynn was given no warnings that he was under investigation, that his answers to the agents' questions might be used against him, etc. Sometimes the famous Miranda warnings are even cited. The idea behind these complaints is that in the governments handling of the Flynn investigation there was a lack of what legal scholars refer to as "fundamental fairness."
The fly in this ointment is that for "due process" and "fundamental fairness" to apply, there is a
threshhold issue: these concepts normally only arise when there is some government process in motion that would deprive a person of "life, liberty, or property." The process may be judicial, administrative, or executive, but some process or proceeding is normally required before a court will consider fundamental fairness. Thus, obtaining a search or seizure warrant (seizure of property or person) is typically the beginning of a due process examination.
The reason Flynn was not provided with any warnings was because the Miranda rules only apply when the person being questioned is either already in custody--has been deprived of liberty to that extent--or the law enforcement questioners intend to take that person into custody upon completion of their questioning. Neither of those conditions applied to Flynn. Flynn was free to show the agents the door at any time during the interview, or to have refused to be interviewed at all. He wasn't in custody of any sort. Nor was Flynn about to be taken into custody. Even had Flynn confessed to being a Russian agent the agents would not have handcuffed Flynn and led him out of the White House. I guarantee you that. Therefore, the agents were under no legal obligation to inform Flynn that they intended--if possible, and subject to the opinion of DoJ prosecutors--to use any statements Flynn made against him. That understanding of due process and fundamental fairness is, to this day, settled law. You may think that that situation offended fundamental fairness, but the courts have the last word and they disagree with you. Or have up to this point.