How does a planetary gear reducer mechanism work?

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The axes of the sun gear, the planet carrier, and the ring gear are coaxial, with the sun gear located at the center of the mechanism. The inner side of the planetary gear meshes with the sun gear, while its outer side meshes with the ring gear. The planetary gear is mounted on a planetary shaft via needle bearings, and this planetary shaft is symmetrically fixed to the planet carrier. In addition to rotating around its own axis, the planet gear also orbits the sun gear along with the planet carrier. Typically, the planet gears have 3–6 teeth that are evenly arranged around the sun gear. The sun gear, ring gear, and planet carrier are all capable of rotating. If only one of these three intermediates has an input, then there is no output; two of these intermediates must have inputs (or their speeds must be determined) for an output to be generated.