It's not really about shaving off weight anymore, as they have constantly increased the minimum weight of car+driver, so weight isn't really an issue. It's the aero, chassis and power pack and engine management electronics that is important. More and more parts are homogeneous, so it's mainly the chassis and engine management that differs. Next year there will be only 3 engine suppliers, Mercedes, Renault and Ferrari, as Honda are pulling out after this season, which is a shame, as they finally seem to have a competitive power pack this year. So a team that buys engines from Mercedes, will have the exact same engine as the Mercedes factory team. Engines are supplied to F1/FIA and them redistributed to the teams. This ensures that the factory team can't supply better engines to themselves. The engine management systems though, are made by each team.