When he confirm one serial number in the work order, Odoo automatic create a new manufacturing order and work order in the background. So you end up with one manufacturing order for each of the serial numbers. Function is the same for lot numbers.
If you look up the right corner of the work order screen, you will see the order number changes for every confirmation: MO/00012, next MO00012-001, next MO00012-002 etc.
In the video they also have a step 2 for quality control, so you see three new work orders that are created for this: One for each serial number created in step1 operation.