[bonzini]

The PCB can be prepared with KiCad or Eagle for example. Then you do a lot of preparation work. You have to prepare the bill of materials, be super careful about the annotation layers on the PCB to prepare a PDF of the components.

Finally you pay. :) PCB assembly services exist from various places, for example Seeedstudio is very reliable but pretty expensive. Note that different factories have different formats for the BOM and annotation PDFs.