How do civilians design their ships?
Can they only use components you've designed or can they make their own?
They design ships based on your researched technology but they use their own template designs and their own components.
There is sort of a flaw in how this work... you should be careful of researching low powered engines and the reasons are two fold. The civilians use the lowest setting and they never pay fuel anyway, this means their ship become very slow and thus deliver less goods per tonnage. The other issue is that the ships become a bit cheaper so they build more of them, even if this to some degree offset them being slower the issue is eventually game performance instead. If you keep to the higher low power research then you get less but more efficient civilian ships which is a win/win situation.
If you still want to use the lower engine settings you can still research them but then removing them with SM and only add them as you design something and the afterwards remove them again and thus you will get less performance decrease from civilian ships as the game goes on. I don't see this as cheating but more as a quality of life type of thing. I usually allow the civilians to build their ships at either 0.4 or 0.3 engine efficiency.