Bearings are what allow the gears to rotate efficiently. They are usually manufactured away from bronze or brass, and so they suit into races (or grooves) inside the gearbox housing. Synchro rings have enamel on them that mesh with the tooth about the gears, plus they also have friction elements https://knoxalsyf.elbloglibre.com/34072121/what-does-filter-gearbox-mean