Manual transmissions I’ve been into do not have reverse synchronizers as they’re not really needed due to straight cut gears and the fact that going into reverse is usually done at a dead stop.
If a transmission grinds going into reverse that’s generally caused by an iffy clutch or a faulty pilot bearing in the flywheel or crankshaft as the case may be.
As to the air-cooled VWs the reverse gearset is internal to the transaxle case just like everything else.