What games let you build your very own ships? Or at least very heavily customize them?
I would love a game like X4 or Starsector where you get to build a lot of AI controlled ships that would let you design them all from ground up.

What are some games that let you design your own vehicles/weapons/units or so?
I always enjoy things like that, but there seems to be precious few games that actually give you freedom like that. I mean stuff like Space Engineers, Avorion, From the Depths, and to a lesser extent even games like Noita that let you design your own spells, or some Mountain Blade mods that let you create your own troop trees.