Weren’t there rumours (maybe two or three years ago) that Lucasfilm or Disney was scanning and digitalizing all SW material they have in the archives? By the look of it, they have indeed - or are still doing it. Just look at all this perfect quality BTS material, especially the crew footage of ILM.
They were. I also wouldn’t be surprised if some of the workers had their own footage as well, bringing in their own home movie cameras and saving that footage in their basements for years.