All humans can access and use Forerunner tech. They can because The Librarian, a Forerunner with a high place in society, felt sorry for misunderstanding the Humans during their war (Forerunner-Human War). She then named them the keepers of the Mantle of Responsibility after the Forerunners died during the activation of the Halo rings (note that Humans were going to be given the Mantle even before the Forerunners, by the Precursors, but that's another story). The Covenant have reversed engineered most Forerunner weapons so they can use them.