@ninetiesloki ✦ — Yep, Sephiroth is from Final Fantasy.
You can find all of his lore if you watch video playthroughs of the game Final Fantasy 7, Crisis Core: Final Fantasy 7, Final Fantasy 7 Remake, and Final Fantasy 7 ReBirth.
I'm going for Rebirth's first, since it's the game I bought initially. Rebirth is a pretty good place to start. And yeah, you should be able to find it all on YouTube.