Who runs the Ethereum code?

Could you elaborate on the governance structure behind Ethereum? Who ultimately holds the reins of the codebase? Is it a centralized entity, a consortium, or a fully decentralized community? How do updates and changes to the Ethereum protocol get implemented? Are there specific individuals or groups with the authority to make such decisions, or does it involve a consensus process? Understanding the governance model of Ethereum is crucial for assessing its long-term stability and potential for growth.