There’s different variations to cover 3. There’s some when the the strong safety rolls over the top of the corner and takes the deep third over the corner who’s responsibility is the flat.
In zone coverage you pass off receivers from your zone onto the next.
If someone runs out of the back of your zone you track them.
The flat has nothing to do with Marco. That's on the LB and you can see the LB pick him up before he notices Burrow is launching it. If he misses he misses then Marco can come down and they get a chunk play. But the supposed CB1 shouldn't stand there covering air while the opposing WR1 gets a running start at your backup free safety.
Then when reporters waited for him at his locker after the game he just ignored them and walked off leaving others to take responsibility. That's chickenshit stuff.