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Other than some ultra lights, balloons, etc, all aircraft in the US are required to be equipped with a transponder that transmits a 4 digit code. Aircraft that are flying by visual flight rules and not under ATC control transmit 1200 as their transponder code.

Aircraft being directed by ATC will transmit whatever code ATC assigns them. For example, an instruction to "squawk" 4732 and "ident" would be telling the pilot to select 4732 as his transponder code and press the identify button which causes his aircraft to be high lighted on the controllers display.

There are 3 codes that are reserved for emergencies. 7500 tells ATC that the aircraft has been hijacked. 7600 tells ATC the aircraft has lost radio communication. 7700 is for all other emergency types, such as fire onboard, loss of engine, landing gear problems, etc.

https://executiveflyers.com/emergency-transponder-codes/