Here's some more context about the upgradeability of the protocol:

Any changes to the protocol occur through Aave Governance. A proposal cannot be executed on ETH mainnet without going through an on-chain vote of the Aave token holders.

Other network deployments will either use a cross-chain executor to relay proposals from ETH mainnet (currently live on Polygon and soon for Arbitrum + Optimism), or if there is no executor then the market is controlled by the Guardians multisig which is appointed through governance.