The thing with Catalonia strikes me as a case of "Arguments sure get weird when the truth is not inside the Overton Window of either side."
Historically, populations have usually been the subjects of governments, owned by them and farmed by them. The Catalan population is the property of the government of Spain. To let subgroups defy this system of property and take possession of themselves and the land on which they reside would greatly destabilize the current world system. I'm not sure it would be a good thing for anyone if states and counties seceded into ever-smaller units and elected the usual run of even more nitwit governments, especially if it was only the democracies that started doing that.
But you can't say that Catalans are the property of the people of Spain, subject to the command of Spain's bureaucracy and control system including its votes of the larger population of Spain in which Catalans will never be a majority, and that Spain has enough force to keep them as that property and we'll pragmatically respect that. We can't just admit we think it's better for the international system if we do things that way in general, so that we have fewer, larger national governments which we think is a public good. That goes against the mythology of democratic self-determination, and governments deriving their power from the consent of the governed.
So instead of appealing to it being best for everyone if the international order stays stable instead of fragmenting into ten thousand pieces, and this making it wiser not to disturb the current tacit policy that governments are allowed to keep whatever population-properties they currently own whether those people consent to it or not...
...people be like, "But the secession referendum is ILLEGAL! According to the CONSTITUTION OF SPAIN!"
I don't think I really understand this, in the sense that I don't think I could pass the Ideological Turing Test of somebody who can think that while keeping a straight face. One government passes a law saying that a population is not allowed to have their own government--and people are capable of getting worked up and indignant about that population trying to get it anyway, because that means for Catalonia to have an independence referendum is ILLEGAL?
I mean, really? This might make sense if they were voting to independently legalize marijuana or something. But the whole point of this vote is exactly to withdraw their "consent of the governed" from the Spanish Constitution and be governed by a different constitution instead. How on Earth are we supposed to be morally indignant about the illegality of that according to the Spanish Constitution? Where on Earth do people suppose a higher legitimacy could come from, to say what constitution is illegal for someone to consent to be governed by? There's superior military force, sure, but to be indignant about the illegality of somebody defying that?
There's more people in the USA than in Spain. Can we, like, amend our Constitution to say that Spain is part of the US and doesn't get to hold separate independence referendums unless the whole population of the US-plus-Spain can participate? Should one then be terribly indignant if Spain tries to hold a separate independence vote when that is ILLEGAL under the CONSTITUTION OF THE US-PLUS-SPAIN which has, DEMOCRATICALLY by a MAJORITY vote of that whole population, determined that the US+S includes Spain?
My best guess as to what people could possibly be thinking is this:
For them, the Law is a reified thing.
The Law in Spain isn't, like, a thing that a bunch of politicians, elected by a Spanish majority of Spain-plus-Catalonia, got together and wrote to govern a bunch of Catalan people. It's not the word and opinion of one government, that thinks it owns a populace, which declares that populace is not allowed to vote to form a different government, backed up by superior military force. It's the Law, and the Law is above everything, even governments.
Also, the Law is whatever currently has police and a military backing it up. That's how we can tell what the Law is, the Law has police. It's not that the police make the Law, it's just a sign by which one's brain detects the Law.
And then that Law is this reified thing floating in the air above everyone, and it can make a Catalan independence vote every bit as wrong-because-it-is-ILLEGAL as jaywalking.
Whereas the USA doesn't have police in Spain right now, so obviously the US Constitution is not the Law in Spain, so it just feels silly to imagine the US claiming that it had some law or other that gave it ownership of Spain. That would just be one bunch of people saying that they're a majority of themselves plus a smaller bunch of people, and declaring they held a democratic majority vote determining that the smaller bunch of people aren't allowed to have their own government. The two cases are obviously completely different! The US+S isn't even an ontologically basic unit that incorporates Spain in an ontologically basic way!
I'm guessing that's what those people are thinking on that wordless level?
But I don't think I could write the internal dialogue of a character who thinks like that, and have them be a convincing human being.