Step one: fabrication
A fabrication shop makes the bare board. It needs Gerber files, one for each copper, mask, and silkscreen layer, plus a drill file. The layer count, board size, thickness, and surface finish set the price.
Check the shop's limits before you design: minimum trace width, minimum hole size, and the layer count it can make.
Step two: assembly
An assembly shop places and solders the parts. It needs the Gerbers, a bill of materials as a CSV, and a placement file that gives each part's position and rotation.
Machines place most parts and an oven solders them. Parts the machines cannot place are fitted by hand.
What usually goes wrong
Shortages: a part may be out of stock or have a long lead time. Check availability before the run triggers.
Fine-pitch parts: tiny packages need tighter process control and cost more to assemble and inspect.
Counterfeit parts: buy from known sources and keep a trace for every board.
In a 01 chain
Fabrication and assembly are two nodes in the chain. The fabricated board ships to the assembler, and the assembled board ships to final assembly and test. Each board carries a serial code that is scanned at every step.