KML files for different cities, countries etc. are often available online. For example, Los Angeles City: https://catalog.data.gov/dataset/city-boundary-of-los-angeles/resource/9f447e3f-959f-4f51-98cd-5fe899c0b898
Tip: The following error will appear if a name field is not specified in the KML file.
If this occurs, use a code editor to open the KML file and add a name tag (Example: <name>Los Angeles</name>) below <Folder><Placemark>. Then, Save.