How do breaches work exactly?

Do I have to kill breach specific monsters in order to keep it open? For now I was clearing the area around a breach not to waste time on monsters that I thought would not fuel it. Is that the case?

I'd love a breach timer to see how I'm actually doing,because some breaches are in places where I can't even find breach monster spawns(tight corridors,walkways,dead ends). Is there a chance to summon the breach boss or is it guaranteed once you reach the 1 minute threshold(it's 1 minute max,right?).

Thanks.