Back in 1942, while we were fighting wars on two fronts, they managed to round up 120K people and put them in detention camps. Removing a million would be child's play with military involvement and indiscriminate detention policies greenlighted by the Supreme Court. Here are some issues to ponder:
1. There are no good remedies for citizens who are unlawfully detained. Kash Patel can wink wink tell his stormtroopers to round up anyone who looks Hispanic and take them to a sorting facility. Anyone who can prove their are a citizen can be released upon proof. So the citizens can be rounded up and then released. But what are the remedies? There are none, not any more.
It used to be that if federal agents violated your constitutional rights, you could sue them in what is known as a Bivins action. The Supreme Court has been narrowing Bivins to the point where it is barely a thing at all, and at the first opportunity presented, they will overrule it. So the detailed Latinos can't sue the officers. If they sue the department, the department will say, "it's just bad apple agents." They might win in court eventually. but it will take a long time and by then the detention/internment/concentration camps will be full. And at most, the government will have to pay damages, which Trump doesn't care about in the slightest.
2. It also used to be that a judge could issue a nationwide order to stop this type of shit. But the right-wingers have been abusing nationwide injunctions to the point where the Supreme Court is basically staying all of them. So there could be some judges in California who would put a stop to this, but their injunctions would only be valid in their districts. There is no way an injunction against this thing would be upheld in the Fifth Circuit, which is Texas, LA and Mississippi. You can round up a lot of people in those areas. I also think the newly radicalized 8th would not consider an injunction, and that's AR, MO, IA and I don't remember where MN or KS/NE are.
3. Trump will declare a national emergency and use the military, for logistical support if nothing else. The Supreme Court let him declare a national emergency to build the wall with funds allocated to the Defense Department. Nothing has changed on the national emergency front. I see no reason why the Supreme Court would change its tune now. And if there are any problems, the Republican Congress will pass a law to allow the military to be involved in deportation actions. There is no constitutional prohibition on the use of the military. It's just a statute.
4. If the courts try to stop it, Kash Patel will largely ignore them. He'll make token gestures at ending the "worst abuses" of the program while maintaining the programs. Will the threat of contempt stop his department? No, it will not. First, Trump can pardon everyone involved. Second, the right-wingers have already shown a willingness to lie and deceive courts. John Gore lied his ass off in the case challenging Trump's census bureau hijinks. He did so knowing that he could be subject to any number of sanctions. He was caught. Nothing happened to him, and again, Trump can pardon.
Well, surely if an attorney were to disgrace himself that way, there would be consequences, right? John Gore is now a partner at Jones Day. He was, to my knowledge, not a partner before he started working for Trump. He was actually rewarded by the right-wing legal machine for doing what he did.
5. If Trump is serious about rounding up millions of people, he could do it and it wouldn't be all that challenging. And while I actually doubt that Trump cares enough to follow through, his underlings absolutely do care and will take it on themselves. Kash Patel and Stephen Miller, for sure. The AG will be a MAGA loyalist and he too will probably be a true believer.
I'm not saying it's guaranteed to happen, but your optimism that it can't is misplaced, imo.