- Sepiroth is cellular and in the lifestream. As long as he exists in a memory, he can live forever and be reborn. Cloud is his anchor. As long as Cloud persists, Sepiroth can be reborn.
- He had forgotten how he looked, so he created himself as three versions (the three from Advent Children. These are the three aberrations you fight in the end of FF7R. The Harbinger is basically Jenova from a future it has total control of fate, but in a future Sepiroth "loses". )
- Aerith has the ability to enter and leave the Lifestream the same as Sepiroth. She knows something is in the lifestream that doesn't want to join it, and is basically corrupting other entities in it with anger/hate making them unable to join the stream. It is the entity that killed her. She contacts other ancients in the stream and while she is able to put a few souls to rest, she is afraid to look at what is causing it. However, she tries once and is chased off, but sees it's intent and Cloud is a part of it.
So, Sepiroth knows what the future is, so he now wants to change it. He guides the party towards the conclusion where they destroy the fate of the future where Sepiroth loses. Theory behind the flashbacks you get when fighting the Harbinger is then that Sepiroth is pushing you towards killing it as he wants you to end a future where he loses. You are being manipulated. You are not fighting Sepiroth from the "original", you are fighting Sepiroth from Advent Children.
Once you kill "fate", it is no longer set in stone that Sepiroth will lose. The end of FF7 was a "bad" ending so to speak. Red hints at that in the remake as well, and the developers have previously stated that the future you see Red running in is a place devoid of humans. They all died.
So the theory they got now is that the timeline for the original is broken. You will still do the "original" content, but with the possibility to now face a change in ending, perhaps through choice. Because to get the "good" ending, to actually end Sepiroth, it's not Aerith that needs to die. It is Cloud.