Yes, it applies to all movements
Unarmored Movement (PHB, 78) States:
Starting at 2nd level, your speed increases by 10 feet while you are not wearing armor or wielding a shield.
It does NOT qualify as your Walking Speed (or Swimming Speed, or Flying Speed)...just Speed. This applies to all types of movements and would add additional speed to the Fly spell as well.
This is supported by Jeremy Crawford as well.
Unarmored Movement is intended to increase a monk's innate speed, including an aarakocra monk's flying speed.
He further clarifies Bonuses and Penalties that apply to Speed in general:
Bonuses/penalties to speed apply to your speeds in general, unless the text specifies walking, flying, etc
As the Unarmored Movement does not specify a specific type of speed, it applies to all.
Fly Spell specifically
Fly states (PHB, 243)
The target gains a flying speed of 60 feet for the duration.
The implication here is that you (the target) had no Fly Speed (or a lower Fly Speed) before casting and it is now a speed that you, the target have. Once it becomes your speed, it becomes subject to Unarmored Movement.
Similar to Special Types of Movement in the PHB, page 182)
While climbing or swimming, each foot of movement costs 1 extra foot (2 extra feet in difficult terrain), unless a creature has a climbing or swimming speed.
Here, Swimming has a penalty, unless they have a Swimming Speed. Once they have that Speed, from whatever source, then Unarmored Movement should apply.
Rule of Cool
And it's fun! Monks are supposed to be super quick. Let them be super quick in all their speed!
Here are the rules on falling from the Basic Rules (or PHB p. 183):
A fall from a great height is one of the most common hazards facing an adventurer. At the end of a fall, a creature takes 1d6 bludgeoning damage for every 10 feet it fell, to a maximum of 20d6. The creature lands prone, unless it avoids taking damage from the fall.
The fall itself wouldn't cost movement (it's "forced movement", except in this case the thing forcibly moving you is gravity). However, if you take damage and thus land prone, it will cost movement equal to half your speed to get up afterwards:
Combatants often find themselves lying on the ground, either because they are knocked down or because they throw themselves down. In the game, they are prone. You can drop prone without using any of your speed. Standing up takes more effort; doing so costs an amount of movement equal to half your speed. For example, if your speed is 30 feet, you must spend 15 feet of movement to stand up. You can't stand up if you don't have enough movement left or if your speed is 0.
To move while prone, you must crawl or use magic such as teleportation. Every foot of movement while crawling costs 1 extra foot. Crawling 1 foot in difficult terrain, therefore, costs 3 feet of movement.
Assuming you have a speed is 30 feet, if you spend 5 feet of movement to step off a ledge, you will take 1d6 bludgeoning damage and land prone. Then it will cost another 15 feet of movement (half your speed) to stand up and continue moving; alternately, you could remain prone and continue moving, but you'd only be able to move 12.5 feet. (Your DM may have you round down to 10 feet if you're playing on a grid map.)
However, if you are for some reason able to land without falling prone or taking damage, then you could move 5 feet and fall off the ledge, land on the ground below, then continue moving as normal.
Best Answer
The Fly spell grants flying movement. How you use that movement is entirely up to you
The spell Fly grants flying movement:
Flying movement is described in the Movement and Position section of the basic rules:
When you have flying movement, you are free to move through the air at any altitude you like.
The only limitation is how far you move. You can move through the air up to your flying movement (which the Fly spell grants 60) at any altitude (or change altitude) so long as you do not exceed 60 feet of movement during a turn.
You may, of course, take the Dash action while flying to move 120 feet in this way. Some classes/races allow you to also Dash as a bonus action, which would increase this to 180 feet. And some affects will reduce your flying movement. (Warding Wind spell creates a 10' radius that is treated as difficult terrain, which would effectively halve the movement of a creature flying through it for that distance).
If you have not moved and are standing on the ground, you may fly up to any height that is less than or equal to your flying movement (ignoring the Dash action).
You continue to fly and may fly back to the ground as long as the spell is affecting you.