R134a fitting installation

[Reddeye]

I’m slowly replacing and updating parts to get my AC working again, and purchased an R134a fitting retrofit kit. Couple of questions for the experienced:
(1) It came with two high-side fittings (different thread sizes) and one low-side fitting, is this to account for different sizes of high-side sizes or is there a third fitting somewhere in the system?
(2) There doesn’t seem to be a way to remove the old fittings. Do the adapters just screw on to the existing fittings, and I just need to remove the Shrader valves from the old ones?