how do they work?
Populated Colonies produce some random amount (per pop) of some things, and demand some random amount (per pop) of some things -- possibly the same things.
Civilian freighters move stuff from colonies where it's exccess to demand over to other colonies where there's a shortfall.
what dictates what a planet or colony will export or import?
Random roll, save that 'Alien Artifacts' are only available from bodies that have (or used to have, depending on certain bugs) Ruins.
do we have any controll over that?
No. . . except for the the SM-only 'Random Ruin' button.
I would imagine a place with financial centres and construction factories to produce a similar tipe of products to one another, and to be different from a terraforming colony...
Yup. It's sort of in the air as a long-term suggestion to modify the current system once other things are "working properly."