The answer here, as with so many things in robotics, is: It Depends.

UR10e robot arm that can lift a 4kg object with a reach of 1m and has sub-1mm repeatability? Strain wave gears in the base and shoulder joints, 100:1 ratio.

MIT Mini Cheetah robot dog that can do backflips? 6:1 planetary gearbox.

Shadow Hand with 20 degrees of freedom? Tendon driven, with the 20 motors in the forearm to keep the fingers slim.

Little dinky Huggingface SO-101? Servo motors, integrating 1:345 gearing with a series of 6 tiny brass gears.

Mid-price CNC milling machine, if you call that a robot? Really long ballscrews, driven by stepper motors.