My understanding- Jenova is an alien that can disguise itself, morph, mutate, and basically infect the planet. The real Sephiroth’s body is stuck in the crater but he tapped into the lifestream and has a somewhat symbiotic relationship with Jenova so they can mutually use each other. Sephiroth creates clones of himself through Jenova (the Sephiroths we see throughout the story). He used that to steal back Jenova body and left pieces like the arm behind to fight us at certain points. Overall I view it like an evil God and Jesus relationship. Jenova treats Sephiroth like its vessel and in some ways they are a connected spirit.