Oh, sorry! No, the vast majority of Final Fantasy pieces are by Nobuo Uematsu. There are specific "piano collections" books you can get with properly-arranged piano versions, which are the ones I played. There are also easy piano score versions of the complete soundtracks to various FF games, though the arrangements aren't quite as good.

I doubt you'd find these online or in an app, to be honest, as they're quite "specialist". I had to import most of them from Japan at moderate expense. I'll have a look though, and let you know if there's any means of getting them electronically.

The composer of the Persona 3 and 4 soundtracks is called Shoji Meguro, and the sheet music for both games is available as a book. I forget where I got my copy from, I'm afraid, but I imagine a quick Googling will turn up a few options that will ship to you.

The slideshow was made using iMovie on my Mac. There's a handy facility in there where you can insert a music track and then mark "beats" in it where you'd like, say, images to change or footage to cut to another angle/scene. Then it was just a case of inserting the photographs, occasionally fiddling with the Ken Burns effect (that's the zoomy-panny effect on the pics) and inserting some transitions between each picture. After that, export and upload to YouTube — I believe iMovie even has the facility to upload directly to YouTube from within the program.

The music was recorded from my electric piano hooked up to my Mac and using a piece of software called Logic Studio. It takes input from a keyboard/electric piano (or notes input by hand/drawn on a stave) and allows you to fiddle with the sounds. I just used a straightforward grand piano sound for this one. After that, you can export the music as a straight audio file, like an MP3 or a WAV, and then that can be brought into something like iMovie to use as a soundtrack.

Hope that helps 🙂