I'm fairly new to Civ and am a little confused as to the differences between citizens and workers.
Citizens make up the population of your cities, so a size 10 city has 10 citizens.
Each citizen can be allocated to a tile from within the city screen and can also be placed inside certain buildings to help produce great people.
If a tile inside the city screen has a green face, you have a citizen working it, if it is grey, you don't.
Citizens can be automatically assigned tiles to work by the city Governor and can also be assigned manually, or semi-automatically by selecting a type of focus, such as food, or production.
Manually assigning citizens allows you to lock them to a tile and are represented by little green locks.
Workers are civilian units that can improve tiles by building farms and mines etc, allowing your citizens to get more food and production etc from each tile.
Workers also build roads and improve luxury and resource tiles, allowing for trade routes and access to luxuries and resources respectively.
When you don't need any trading posts/farms etc.., what is the benefit of using the automated setting for the workers? What do you gain by having a plethora of workers, on the automated setting?
The benefit of automation is that workers will generally hook up luxuries and resources as soon as they become available to work without you having to find them and manually assign a worker.
However, the AI that controls worker actions is fairly stupid and will often build an inefficient road network, which wastes money, get themselves captured by enemies and try to build roads to city states half way across the world, which usually wastes money and gets them captured all in one.
General best advice is to always manually control your workers.
If a tile in your city is showing 2 food, does this still produce this even when no citizens are working the tile? If so, what is the benefit of placing a citizen on that tile? I assume it increasing the production, but am a little confused.
If a tile doesn't have a citizen assigned, then it's not contributing to the city that it surrounds. Each citizen requires 2 food to live, so assigning a citizen to work a 2 food tile will not make the city in question grow more. For growth, a tile needs to have in excess of 2 food worked by a citizen, unless you have maritime city states providing food.
A tile with 2 production and no food will provide 2 production when worked, but other tiles (or maritime city states) will need to cover the food cost of the citizen working that tile. In a way, food equals production, as you can use excess food to assign citizens to high production, zero food tiles, which you couldn't do without excess food, without starving a city and reducing it's population.