Bearings are what allow the gears to rotate efficiently. They are typically designed from bronze or brass, plus they match into races (or grooves) from the gearbox housing. Synchro rings have tooth on them that mesh with the tooth over the gears, plus they even have friction products on them https://rylantentb.thezenweb.com/the-definitive-guide-to-filter-gearbox-72118530