Many of these industrial systems are built from reusable components. In my experience at a Tier 1 OEM for the US domestic auto market, a line could be torn down and re-purposed to another line by adding or removing physical components, modifying the physical layout where appropriate, and then adjusting the PLC programming.

An assembly line isn't really a "product," per se. It's more of amalgamation of several products and technologies, many legacy, that forms a cohesive but loosely-coupled whole. You may have several vendors' products / systems in a line. So what you end up with are ongoing contracts that provide for installation, programming, troubleshooting, maintenance, etc.

There's actually a whole specialized industry that does nothing but design assembly lines. It's pretty neat.