Does lvl 10 Flurry of Blows cost 2 focus?

I am struggling to parse monk's level 10 class feature, Heightened Focus, at least as it applies to Flurry of Blows. Is this feature just upgrading Flurry of Blows, so now for one Focus and a Bonus Action you get three attacks? Or is this providing an optional upgrade to regular Flurry of Blows, which costs an extra Focus point? Basically, for a level 10 Monk does a Flurry of Blows that hits three times cost one Focus point, or two? How would this interact with Shadow Flurry from Cloak of Shadows?

P.s. I apologise if this is very obvious to some of the other players here, this is my first character and my first time playing DnD so I'm trying to figure things out.

Thank you! I am really excited to play for the first time 😁