What is a bike kit? What are the common kits?

Composition of bicycle
A bicycle usually consists of 3 parts:
Frame (with saddle, handlebars, frame, fenders—everything attached is almost stationary).
wheel
kit

A kit is the sum of the components of a bicycle's transmission, gears and brakes. Usually includes front sprocket, chainring, chain, rear sprocket, brake, brake and shift lever, derailleur. Sometimes hubs are also included, although the term is strictly used to exclude hubs.

Components of the kit
Dental plate
flywheel
chain
derailleur
transmission
brake lever
brake

The crankset consists of bearings and crank arms. The pedals and chainrings are attached to these bearings. A good chainring should have a stiff crank, good quality bearings and a solid chainring.

Flywheel - rear sprocket: The flywheel is composed of a set of rear chainrings. Expensive flywheels are lighter, and they are generally non-steel materials.
Chain: The chain plays the role of transmission. Generally speaking, the harder the chain, the longer the service life. However, there should be a balance between price and weight, if it is more than a point, it is of little significance.

Derailleur: Used to change gear ratios. They move the chain from one sprocket to another. Usually road bikes have front and rear derailleurs, while mountain bikes are more inclined to eliminate the front derailleur.

Derailleur: Used to control the derailleur to adjust gear ratio changes. Sometimes they are integral with the brake lever, sometimes they are separate.
There are two basic shifting types:

Friction Shifter The rider pulls the handle until the gear shifts.

The index shifter is shifted by pressing the button, or moving the handle and hearing/feeling it to click.

The brake lever is used to control the brakes. The good ones are usually made of aluminum or steel, while the cheap, bad ones are usually made of plastic.
Brakes are used to control how fast the wheels turn. A good brake pad should have these two properties:
a) Strong braking force
b) Good handling