According to the wording of the fly spell:
You touch a willing creature. The target gains a flying speed of 60 feet for the duration. When the spell ends, the target falls if it is still aloft, unless it can stop the fall.
The wording of the spell says that the creature "gains" 60 feet, so I am curious if this stacks with the natural fly speed of the creature. Let us say a creature has a fly speed of 30, would that creature then get 90 feet per turn, or would it go to 60 feet?
Additionally: If a creature with a fly speed has the fly spell on them and concentration is broken, would they technically fall, if the flight speed is solely dependent on their use of the spell?