I think it's pretty clear/logical. Your stuff is all on the left, so the stuff you give up goes in the left box. That's not to say the interface is perfect. The ship design menus need the most work, IMO. There needs to be some way to sort and filter all the designs. I'd really appreciate an "upgrade design" button, where you update the components, then it's automatically named and saved as a successor to the previous design, which is now hidden from view. Then, next time you play, the base design will automatically be replaced with your upgrade once you get all the requisite technology.
Actually I'm not so sure. Declining this, going to the diplomacy screen, this minor race CANNOT offer "Open Borders" which leads me to suspect they're offering 608 bc to receive Open Borders from me.
Again, there should be explicit "they're offering" / "they want from us" labels.