how to build alien ships?

[Jorgen_CAB]

But if I can't build ships without these components then the number I can build is limited and thus I'm better off extracting minerals out of these components and building my designs.

This is entirely up to you... the game currently work as said. You don't get any advantage from alien components from which you already have the necessary main technology. You still have to design such a component yourself and spend the research on it. You are mostly better off by taking the minerals.

I would agree that if you have the main technology you should perhaps automatically get some points in the component tech if you disassemble the them. So you can research those components for less RP, but that is not how it currently work. You could make this a suggestion in the suggestion thread though.

[Black]

I usually scrap salvaged components. You may decide to build some special ship if the components are much higher tech-level than your ships. But it does not seem that useful to retool or build  shipyard for one or two ships.

If I manage to capture larger number of same alien design (that would be most likely Precursor ships). I sometimes make partial refit to keep them in service with my navy. In my current game I had mostly missile ships, so captured beam ships (railgun and gauss) were useful boost for point defence or jump point assault role and I did partial refit with shields. Engine refits are also possibility but engines are quite costly so it depends on your situation.