If there's anything the Victorian era is known for, it would be trade, and the systems built around it, such as weaponizing it for political gain. In the most recent dev diary, developers explain how that works exactly.
In Victoria 3, nothing works on its own, especially not the Institutions and buildings, so the trade centers must be staffed. Goods do not just appear in one nation from the next but require the maintenance of bureaucrats, laborers, clerks, and the like to offload and unload cargo, take account of it, tax it, and move it forward, developers explain in their latest dev diary.
Without pops staffing your trade centers, they go on to say, players will find themselves unable to conduct trade in accordance with their aspirations. Luckily, staffing these shouldn't prove to be incredibly hard, as trade centers are, usually, hot spots for migration.
These trade centers collect revenue on both sides of the routes they manage, which is related to how many goods are moved as well as how much the routes affect the prices in their respective markets. This revenue is allocated to the employees and taxed by the same logic as any industry, so who makes money off your trade is related to your domestic policies in the same way as the rest of your economy, which is governed by your country's Laws.
And, while trade is something every nation can take a part of, how they affect trade, whether it's preventing it when it hurts them, or profiting off of it when they can, is dependent upon said country's trade policies, something that also dictates tariffs and embargoes, as well.