What city in the United States uses the area code 800?

800 is the toll free area code - it does not belong to a specific city or area.

https://www.mightycall.com/800-area-code-number/

800 is the original designation for WATS numbers or Wide Area Telecommunications Service. There were Inward WATS and Outward WATS lines with six different levels of calling areas, level 6 being the entire United States (all 50 states) level 5 being the 48 contiguous states, all the way down to level 1, only one state. Usually a company will get a level 6 and then a level 1 because even though level 6 allows customers to call from anywhere in the US, it excludes the state where the call is terminating, so the company usually buys a level 1 for that state. Interstate Inwatts are less common but still exist. These are the ones you can only call from one state. Calls are billed by the hour rather than individually. Area codes 888, 877, and 866 were added later.
https://en.wikipedia.org/wiki/WATS

The North American Numbering Plan Administration maintains a website detailling all the area codes in use in North America and the Caribbean

http://www.nanpa.com