You're right that land ownership is more complicated. It always belongs to the state (I forget the fancy term for it but it's like 'sovereign ownership' or something like that).
You can buy exclusive exploitation rights to this sovereign property but the state can get it back upon your death if it's not passed onto anyone or otherwise accounted for.
And then there's eminent domain, which is actually specifically provided for in the U.S. Constitution so it's about as legal as things get in the U.S.
It's the difference between "ownership in fee simple" and a "freehold lease" or "freehold tenancy". I'm not aware of any ownership in fee simple in the US or Canada; it's usually the sort of thing that a Marquess or Duke has in a feudal sort of arrangement (you own the land outright rather than merely at the pleasure of the sovereign, be that a monarch or The People writ large, but you're required to defend it in turn). Smaller fee simple properties exist in England as well, often as the result of the grant of peerage in ye olde dayes (the Norman and Angevin periods mostly). For the most part, though, in the english-descended world, the most you can have in a property is freehold tenancy.
You can buy exclusive exploitation rights to this sovereign property but the state can get it back upon your death if it's not passed onto anyone or otherwise accounted for.
And then there's eminent domain, which is actually specifically provided for in the U.S. Constitution so it's about as legal as things get in the U.S.