> the people of the Isle of Man were EU citizens.

Not quite, at least not by default. The pre-Brexit Manx passport did confusingly include the text "European Union" on its cover, but holders of Manx nationality were only citizens of the EU if they had lived for at least 5 consecutive years in the UK or if they descended from a UK parent or grandparent.

Source: https://en.wikipedia.org/wiki/Isle_of_Man-variant_British_pa...

It's the same for all the non-EU French territories. EU citizenship is not territorial, it's directly linked to the person's nationality so you wouldn't remove EU citizenship from a French citizen just because he happens to live in New Caledonia. French or British citizens living in Canada also are EU citizens after all.

The state of British citizenship is a bit more complicated though I think. A bit like US citizenship which kinda depends on which US territory one lives, as far as I understand.

No, a British citizen is a British Citizen wherever you live. There are tax implications based on where you live, but that's unrelated.

Right, but citizens of British overseas territories, crown dependencies and whatever other statuses exist are not necessarily British citizens. Same as with the various US territories.

This is what I meant when I said these citizenships work in more complicated ways than the French one.