The constants in the code
const long interval_1 = 337; // ON time
const long interval_2 = 1000 - 337; // OFF time
determine the time the LED or buzzer is ON (interval_1) and OFF (interval_2).
To have something blink once per second, the ON time plus the OFF time should be 1000 ms.
The simulation linked in #6 keeps time with the simulation clock. Better names for the variables previousMillis_1 & 2 might make the logic clearer. interval_1 & 2 also.
Maybe like wentOnAt / wentOffAt and stayOnFor / stayOffFor.
a7