In the OG Cetra were just humans who hadn't lost their ability to commune with the planet. To put it another way, humans are just descendants of Cetra who chose to give up the nomadic way of life and settle down. It's the old Cain v Abel story: God favours the nomads, and turns his back on the settled agriculturalists.
Given that every living thing is animated by lifestream, there's a sense in which every living thing is simply an extension of the planet; that the planet is a single organism and the lifeforms are its cells. Cetra, I assume, had the ability to hear the lifestream within them, and so were able to live cooperatively with the planet. When humans lost this ability they became 'rogue' cells, like a cancer. Shinra is sucking the life out of the planet, like a cancer, but when the planet-organism dies, so will the cancer, so Shinra is digging its own grave.
Anybody can hear the Planet if they know how to listen to the lifestream within them.
Dead Cetra don't normally live on as distinct consciousnesses and wills in the Lifestream. Aerith is unique.