How does the Web spell burn away?


The web spell description says:

The webs are flammable. Any 5-foot cube of webs exposed to fire burns away in 1 round, dealing 2d4 fire damage to any creature that starts its turn in the fire.

I’m confused with the wording of the flammable part.

Let’s say that on turn 1, I cast web; the web is supported by the floor and the ceiling that is 15 feet up. I catch a Medium creature on it and it fails its saving throw.

On turn 2 I cast fireball engulfing the whole web on its radius.

The wording says that “Any 5-foot cube of webs exposed to fire burns away in 1 round” – so, what exactly happens?

  1. What does “Any 5-foot cube of webs exposed to fire burns away in 1 round” means? Does any area of the web affected by fire will vanish and free whoever was caught on that area after 1 round?
  2. If the answer for 1 is yes, what if I use a single-target spell like fire bolt or chromatic orb, will the creature still free itself?
  3. What does “in 1 round” means? At the start of round 3? At the start of my third turn? At the end of my third turn?