Hello! That is a great question to ask. First off, are you familiar with the concept of an interference fit or press fit? For these circular, smaller diameter shafts, press fits are often used to transfer the torque to the pinion. West Coast Products offers gears and pulleys that can be pressed directly onto the shafts of BAG motors and 775Pros. But any gear or pulley with the correct ID (inner diameter) can be pressed onto a motor shaft.
Also, one of the most common ways FRC teams integrate BAG motors and 775Pros into their robots is by using the VersaPlanetary gearboxes by VexPro. These gearboxes use a slightly different method of transferring the initial torque from the motor shaft, but they still provide an excellent way of reducing the motor’s speed to a usable level in a compact and rugged package.