On the economics screen, industry tab, you can select "ship components" and start prefabricating parts for your ships, that your shipyards will use when building ships (and thus speed up construction). I usually build engines in large numbers because, for one, most ships use quite a few of them and two, tech advances not as fast as with other stuff, and I´d hate being stuck with a lot of outdated parts, that I can scrap for only 30% return
I keep finding out new things about this game. Do the shipyards automatically use the built parts, or do you have to tell them somehow?
They will use them automatically. This is a good way to reduce the build times of large, expensive ships. For example, if need terraformers quickly, build the terraforming modules using planetary industry so the shipyards don't have to build that portion of the ship.
I've also noticed (having salvaged a number of wrecks) that I end up in the Class Design screen with parts with a (D) in front of them. Are these parts from the wrecks? If so, doesn't that mean that if I design a class with them, that I can only build so many ships of that class?
(D) indicates a component that you have available but can't build yourself. This is usually from salvage, scrapping or ruins. You can design classes using this component but can only build ships of that design as long as you have the components in your stockpile. For example, if you found six advanced gravitational sensors, you could design ships using those components but once the six components had been used by your shipyards, you couldn't build any more ships of that design until you found more of the component. A big decision you may have to make when you find advanced components is whether to incorporate them in your ships or disaasmble them for potential tech knowledge.
If you find your list of components in class design is being cluttered by (D) components you can either hide them all using the 'Own Tech Only' checkbox or hide them individually by using the Obso Comp button (which sets a component as obsolete).
Steve