wait, let me as some a couple of questions
-did the trade fleet finish loading their goods?
-did you destroy their cargo ships?
for the 1st, if yes, then you should have gotten some of their goods. Which depends on your scaving ability and your fleet's scaving ability (# of ships with salvage gantries).
if no, then there is no salvaged to be have since they are still loading their cargo.
For the 2nd one, if yes, then their goods should be in the vacuum of space waiting to be salvaged properly. If no, then there's your issue. You let the ships carrying the cargo get away.
Industry functions aren't completely hidden from the player. For example, harvested organs are wanted in colonies for "medical" purposes. The higher its population, the higher the demand. It's not like humans can grow another liver if they want to. Food is only needed when there is a shortage caused by different reasons (e.g spaceport disrupted, convoy raids). Usually, food is easily acquired by many colonies via "in faction supply" or are produced locally. There are shown in the "colony info" tab. You can access it via colony>market>press m. It should show you what does what (putting your mouse over the industry) and what needs what (a small window in the right where there are some item icons). You can also check of which markets needs more via pressing F1 when you mouse over an item.
Colony demands depends on what industry that colony has. Smaller colonies requires less since its industries are smaller. The bigger the size of the colony the more it will need. It doesn't really depends on the colony's tech level, but I may be wrong on that.