There really isn't a mystery to it, they just got the roster in better shape. They bolstered thin positions with veteran depth like Torrey Smith, Alshon Jeffrey, Chance Warmack, and Chris Long. They made some shrewd trades for Ron Darby and Jay Ajayi. Howie Roseman knew it would take 5+ years to draft his way out of undoing the mess that Chip Kelly created, so he paid the price in free agency. Roseman is a good GM and made good picks in free agency, he didn't go hog wild signing every bum available. They got lucky to some extent also, they got good LaGarrett Blount and not the one Pittsburgh had a few years ago and that guys like Torrey Smith and Chris Long still had some gas in the tank. Roseman also hired a good coach in Doug Pederson who hired good coordinators in Scwartz and Reich.
GB has a lot of talent, but they also need to bolster depth with some proven vets. Sign and draft a FS, an ILB, a CB, and a TE and this team is as good as any other team in the league. A better backup QB wouldn't hurt either.