There's no need to complicate static sites with jQuery. All it does can be achieved with vanilla.
I mean, all you really need is a Turing Machine, but they're not very pleasant to program.
Same thing with vanilla JS and jQuery.
Well, yes. Every JS library can be written in vanilla JS. The point of jQuery, React, etc. is to make it easier for the programmer to reason about what is going on while avoiding bugs.
I recently had to rewrite a bunch of vanilla promise-based code to use jQuery promises once I realized IE11 didn't work.