A mouse is built around one part: the optical sensor. Almost every serious mouse uses a PixArt chip (the PAW3395 is today's workhorse, the PAW3950 the new flagship at 30,000 DPI), and the sensor sets the tier the way a Bluetooth SoC does for earbuds. Around it sit a microcontroller, the click switches (Omron, Huano, Kailh or TTC micro-switches, the same Dongguan houses from the switches page), a scroll encoder, the shell, and for wireless a 2.4GHz radio and a battery.
Gaming is where the design lives. The race runs on three axes: weight (sub-50g shells in honeycomb plastic or magnesium alloy, on PTFE feet), polling rate (1,000Hz was the standard, 8,000Hz is now the esports bar, a 0.125ms response), and wireless latency (2.4GHz dongles tuned to feel wired). The office mouse, by contrast, is a commodity: a cheap sensor, a silent switch, a long-life AA.
PixArt now licenses its flagship sensors widely, and that broke the apex's monopoly. A wave of China-born brands (Lamzu, Pulsar, VXE, Attack Shark, WLmouse, Darmoshark, Incott) sensor-match Logitech and Razer at a third of the price and iterate every few months, shipping firmware fixes in days. Many are an ODM's own label; the same shell and sensor goes out under several names. The factory designs it; the brand is the buyer.
Compliance is light, CE, FCC and RoHS for the radio and materials, with the real scrutiny coming from the community's sensor and latency tests on YouTube. Above this layer sit the parts you cannot hire: PixArt in Taiwan for the sensor, and the apex brands (Logitech, Razer, Zowie, SteelSeries) themselves. The hireable houses below make for everyone else.