How do mods affect our ships?

Pure theory here, but isn't their affect only on "crew power"? If you switch in a mod which raises the char raiting, it raises its crew power, and thus the strength of the ship. Beyond that, I don't know. I have no idea where speed, attack, etc. of ships come from. There are numbers in the stats saying how much comes from the crew, but I don't know how you can change that (beyond just making crew generally stronger).