Civilization VI uses a Trade Route system similar to previous games. Trader Units can each create and sustain a trade route between two cities. What a Trade Route yields will vary based on what Districts a city has, and depending on the duration of the route will also vary depending on the distance between the two cities (with shorter routes ending sooner).
One of the new features is that a Trader will create a Road (different depending on the Eras) as it travels along its route, so it's worthwhile to have different Trade Routes set up to build routes through your lands.
It seems as though Trading Posts are automatically created in cities in which you've established a Trade Route, and these will add Gold to subsequent Trade Routes which pass through that city.
Internal routes now provide food and production yields based on what districts are located in the destination city. The city center itself provides +1 food +1 production. Theater Squares, Holy Sites, and Entertainment Complexes provide +2 food. Harbors and Industrial Zones add +2 production. Commercial Hubs and Encampments provide +1 production.
For example, City A sends a route to City B in the same civilization. City B has a holy site and a harbor. City A would then receive +3 food, +3 production per turn.
You can create one Trade Route for each Commercial Hub or Harbor district in your civilization. The Merchant Republic Government allows 2 extra Trade Routes, there are several Great Merchants that give additional Trade Routes, and there might be more ways to gain additional Trade Routes.