How does Manny go from chauffeur to

It's more a producer/Studio exec who has his fingers in all different pies then director. A bit like Nellie (Margot Robbie), Manny is in the right place at the right time. He definitely works his ass off as you see for the first hour of the film. Especially helping get the camera for the last shot of the film Brad Pitt's character was filming. He also hit it big thanks to teaming up with Sidney Palmer the black trumpet player and making successful short films with him (something the film definitely struck out on was not building up that relationship/friendship a lot more when it eventually breaks down, doesn't have the impact it should)