I'll try and go through the up-air as systematically possible
Firstly what is the move? Up air has two attacks, the first attack is weak, sparkly and probably an attempt to allow an opponent to SDI out of the second attack (not tested, can someone verify?), this second stage is an explosion with plenty of good properties.
Properties:
Hurtbox - A hurtbox is a hitbox that can damage your opponent but cannot be damaged by your opponent. This allows it to extend upwards safely to challenge moves like Link/G&W/Ganon/and many more DAIR.
Size - This move has a big explosion, so you don't need to be ultra precise with it, and is the other reason it can deal with DAIR's.
Sweet vs Sour - The second attack has two hitboxes, the first deals 14 dmg, the second does 17 dmg and hits for more knockback
Uses:
Platforms
Coverage
Combos
Platforms
Zelda is not a vertical character for good reason, her lightness and slow fall speed means she dies off the top very easily, usually she needs to put herself in a horizontal position to deal with platformed players. Using SFFL up-air however, allows for platform chasing while staying safe from getup attacks or other grounded moves. Particularly the higher platforms on stages like Battlefield, this move is a great asset. The downside is the long ending lag for this move, meaning a miss will allow an opponent to reset the situation very easily, nevertheless it is a solid platform option as a failed LK could lead to a punish, a failed up-air is quite safe.
Coverage
This point is a small yet important one. We all like going for lightning kicks when someone is airborne, but too often we get spotted and they jump over you. "HAHA I have you now" you can say as you simply jump into upair and hit them anyway. This allows for some opponent conditioning, and leading into some followup that I will disscuss now
Combos
Up-air is primarily good for comboing into itself, and really shines against anyone around Ike's fallspeed/weight combination all the way up to wolf. Importantly spacies will not be comboed at 0% with up-air, but if they are in the air you can use up-smash to tack on % till they get up-air comboed. Against the lighter characters it it still very important, and while it doesn't true combo, it can be used to get them as they try to come down.
The second move it combo's into is LK, the DI of the opponent can mean they escape the up-air chain and fall right into a FAIR or BAIR. This can turn an up-air string into death
To get a up-air combo going it depends on what character you are fighting. Throws are a good start, and up throw to up-air against heavy characters is great. the other option is force them on a platform and follow their tech with up-air. Good choices to get them on the platform would be Dthrow or up throw, F-tilt can also work, even up-tilt at low enough percent, anyway, whatever you use to get them on the platform, an up-air can be a great follow up.
That's pretty much it for up-air. A really strong move for killing, and comboing. Anything I missed?
Teach fox how to kill off the top in style.