How do you suggest enabling these use-cases for users that want them, on top of a browser that isolates tabs by default, without requiring any technical aptitude of the user to understand what a "cookie" is (which would normally suggest "have a UX flow for it") but while preventing arbitrary websites from triggering such a flow and thereby tricking these non-technical users into enabling the site to track them?

It's not like the problem is impossible; nobody is saying that. My point is that the solution is non-obvious to the point that nobody has solved it yet, despite likely man-years being put into trying. Firefox Containers are the best UX we've come up with so far to kinda-sorta solve the problem. Do you have any better idea?