The headsets are active between plays, and have one way communication with one player on each side. Typically this is the quarterback on offense and a team captain/play caller on defense. These players wear special helmets typically marked with a green dot on the back.

The refs or other officials cut off communication when the play clock reaches 15 seconds, preventing the kind of real-time communication you suggest.

No. The headsets are disabled when the play starts or when the play clock goes below 15 seconds.

Here’s a really good podcast episode that includes that question:

https://www.20k.org/episodes/blue42