Is jQuery unnecessary because the functionality is built into js now? Or browsers? Or other libraries? If it's libraries, are those libraries providing a superset of jQuery or are they just better?
Yes, the DOM API's have greatly improved and are well-supported among browsers now, so jQuery is no longer needed for that. React doesn't solve the problem of inconsistent and unwieldy browser API's (which isn't a problem any more), but of a clean architecture for web applications.
jQuery is not that necessary anymore because difference between browsers' js implementations are not that big. You still have small differences in events but nothing like it used to be