My guess is that you're assuming all user defined types, and maybe even all non-trivial built-in types too, are boxed, meaning they're allocated on the heap when we create them.
That's not the case in C++ (the language in question here) and it's rarely the case in other modern languages because it has terrible performance qualities.
I think usefulcat interpreted "std::vector<int> allocated and freed on the stack" as creating a default std::vector<int> and then destroying it without pushing elements to it. That's what their godbolt link shows, at least, though to be fair MSVC seems to match the described GCC/Clang behavior these days.
