"The tower cleared JetBlue 1554 to depart on Runway 04, but simultaneously a ground controller on a different frequency cleared a Southwest jet to cross that same runway, putting them on a collision course. Controllers noticed the conflict unfolding and jumped in to yell at both aircraft to stop, avoiding a collision with about 8 seconds to spare "

As a regular passenger and for someone with a little bit of drone autonomy experience, the fact that this is possible sounds insane to me. I just assumed there was something blocking the controllers from putting two flights on the same pathway (like little planes on a board?).

My main question is, how does the "noticing" work now? What did the Controllers see that alerted them to what was unfolding?