Who Does Football Coach Talk To Who Does The Coach Talk To, Through His Headset, In A Game Of American Football?

Who does the coach talk to, through his headset, in a game of American Football? - who does football coach talk to

My son, 11-year-old wonder who can hear. Is the captain, with a headset radio in the helmet or the team? Or is it the referee? Why use this helmet, if not talk to someone?