In Final Fantasy VII, wherever Cloud and company go there is Sephiroth, but as a matter of fact it's not Sephiroth's real body. The headless Jenova stored in the Shinra Building, is the one who manifests in Sephiroth's form.

As for Sephiroth's true self, when he fell inside the Nibelheim Mako Reactor, it dissolved in the Lifestream. Over the course of five years his body reconstructed itself in the Northern Crater where the Lifestream concentrates as he bided his time for his revival.

With the power of Jenova, Sephiroth is able to control the Sephiroth Copies, who were experimental products of Hojo after the Nibelheim Incident. Before long, Sephiroth was able to call Cloud and Jenova's body together to unify in the Reunion, and use them to obtain the black materia to summon Meteor.

His fall into the Mako Reactor
Intentional or Beyond his Control?

Sephiroth ends his life as a human being in the Nibelheim Mako Reactor, but the interpretation of how it happens is divided between the titles. In Final Fantasy VII, despite having being stabbed, Cloud unexpectedly fights back and throws him into the Mako Reactor.

On the other hand, in Before Crisis and Lost Order, Sephiroth jumps into the Mako Reactor on his own volition. But whichever it is, Sephiroth gaining a vast amount of knowledge in the Lifestream and being revived five years later stays the same.

This issue is something that has been debated time and time again. It doesn't help that the source material presents two different points of view on the Nibelheim incident.