The outcome of McGirt v. Oklahoma was that Jimcy McGirt was due a trial in a federal court, not in an Oklahoma State. It can be difficult to understand why this is so. I have read most of the work of historian Francis Jennings, whose books usually cover Native Americans. This reading of history provides highlights to the law that are often buried in legal abstraction.
The precept that natives are due federal trials, only for certain crimes after the Major Crimes Act of 1885, in my mind originates with The Treaty of Ghent with the British after the War of 1812. A somewhat obscure upshot of the Treaty was that the US “preempted” the rights of the British with respect to the Indian nations. Supposedly by the spoils of war with the US preempted the British rights over the Indian nations who were dealt with as conquered peoples under international law. Therefore henceforth all relationships between the Indians and the US were between the Indian Nations and the Federal Government, not the states. This is why McGirt was due a federal, not just a State of Oklahoma, trial.
But there is a problem with preemption: the only tribe that ever surrendered to Britain were the Narragansets. What preemption really brought us was a mass of treaty obligations under the Articles of Confederation. Article 6 of the Constitution still binds the United States to the Articles of Confederation and the mass of treaties.