Why people mention Ethereum as a 'greener Bitcoin' baffles me. There are already some networks that are a thousand times greener than what Ethereum might be when it switches, and that is if the switch goes as planned and on schedule. There are a couple of networks that are already carbon neutral or even negative. If Ethereum takes over Bitcoin, it will still be a wasteful network, and still seen as (rightly so) a villain.

How carbon negative network work?

Same way as any industry/business that consumes energy can become carbon negative.

Ethereum has been "about to switch away from PoW" for years and years. They get zero points for this up until the day it actually happens. For now, they are massively harmful, and judging by their history, will continue to be so.