How do imports work if you don't set up a trade route?

Tropico 5, like Tropico 4, has trading. It does it in an obscure way, though: you can set up explicit trade route, or you can just trade at the default prices for everything. It's unclear, though, how imports work – when does a ship decide to import? Can only your trade ships import, or can your "export" ship also import? How much does a ship decide to import? And why are there trade routes for me to import goods at more than the default price?

I had a Rum Factory create goods on a map where I had no Sugar Plantation, but it did so in lumps of 1000 goods. Does this mean that I'll only import exactly 1000 goods per ship, and only when the factory is empty? Or can I get goods if the factory still has some import goods, but has no other source of input goods? Does having additional Docks (and thus trade ships) increase the rate at which I import goods?

It's nice to know that I can import goods, but knowing the mechanics will help me decide what to focus my effort on.