Other countries have very large cities as well. Take one example - greater Toronto has about 6 million people. And Canada isn't a dictatorship. And yet they have their case rates down to the point where Canadians are now allowed to travel to Europe, where there are also large cities.
The US does have quarantine laws to prevent the spread of communicable diseases - they were last really used in the 1918 pandemic.
https://www.cdc.gov/quarantine/aboutlawsregulationsquarantineisolation.html
The federal government derives its authority for isolation and quarantine from the Commerce Clause of the U.S. Constitution.
Under section 361 of the Public Health Service Act (42 U.S. Code § 264), the U.S. Secretary of Health and Human Services is authorized to take measures to prevent the entry and spread of communicable diseases from foreign countries into the United States and between states.
The authority for carrying out these functions on a daily basis has been delegated to the Centers for Disease Control and Prevention (CDC).
For whatever reason, the CDC has been completely invisible in this pandemic. They are not even allowed to hold press briefings.