Confusion about adding an element to a container being accomplished via construction in place or via assignment [closed]

In Item 41 from Effective Modern C++ Scott Meyers mentions this difference and its impact on the efficiency of emplacement with respect to insertion. I have some doubts about that, but before asking questions about that I need to understand what the difference between these two ways of adding an element is.

Given the code sample from the book

std::vector<std::string> vs;
// add elements to vs
vs.emplace(v.begin(), "xyzzy");

(and leaving aside the fact that all preexisting elements in the container have to be move assigned/move constructed/copy assigned/copy constructed for the all combinations of noexcept-ness of move operations and v.size() == v.capacity()-ness of the container) what are the two scenarios?

• If emplace adds the element via move assignment, it means it does v[0] = std::move(std::string{strarg}); // strarg == "xyzzy", right?
• But what is the other case of contructing the element occupied by v[0]? Is it a placement new? And how would it look like? I guess it should be similar to the chunk of code at the section Placement new here, but I'm not sure how it would look like in this case.