Each city in Civilization VI is comprised of districts, which are specialized areas of a city that are placed on tiles surrounding the city center and provide certain bonuses, such as the campus, which grants science. There are a total of 13 district types, including the city center, each with a different role and a different bonus, which can be affected by terrain.
The number of districts in a city is limited by its population: For every ~3 population the city gains, it becomes eligible to house a new unique district. Players gain the ability to construct districts after they research certain Technologies or Civics, and they may have a reduced cost or be modified based on on what civilization is attempting to build one (such as England’s Royal Navy Dockyard). Once constructed, the district can house up to 3 buildings, which also have prerequisite Technologies or Civics. Buildings can be built in every district, including the City Center.
After the district has been built, players can begin populating it with buildings. Buildings can be built through the "Production Menu" similarly to how the district was built. Some buildings have prerequisites, such as other buildings or technology or civic requirements; these buildings will not be visible in the Production menu. Some buildings are available to build before any districts are placed, such as the Monument. These buildings reside in the City Center.
All Wonders are built on Tiles surrounding the city center. Wonders will be shown in gradual states of being construction as work on it progresses. Once it is finished the construction will be repeated with a cinematic, complete with dramatic lighting of a day going by. Wonders can't currently be pillaged or destroyed, except perhaps by nuclear weapons or by razing a city, though more information on that will be known as the game comes into release.
The term "unit" refers to anything that can move around the map. There are a number of types of units in play – land combat units, naval combat units, civilian units, support units, and the like. Land and naval combat units form the bulk of available units.