What is assembled on a assembly line?

Answer 1

A series of workers and machines in a factory by which a succession of identical items is progressively assembles
A system of manufacturing where the product moves past several workstations in succession and at each workstation a human worker or a machine performs a small number of simple assembly steps before the product moves to the next station. This requires minimal employee training and usually improves both production rate and product quality.

Prior to the assembly line either a small group of workers assembled the entire product from beginning to end and had to be highly trained and very skilled workers, or individual workers carried their tools around the entire factory from product to product to work on each one. Both of these were expensive and inefficient.
It's where products are assembled in a factory. All of the components are fitted together by machines and/or workers as the product moves along the line.
Assembly line is a noun. It's a process where a combination of workers and machines assemble a product step-by-step in a methodical way. It is sometimes referred to as a production line.
An assembly line is a manufacturing process in which parts (usually interchangeable parts) are added to a product in a sequential manner using optimally planned logistics to create a finished product much faster than with handcrafting-type methods.

Answer 2

An assembly line is a system of workers and machinery such that a product is assembled by a means of consecutive operations.