How does a UUPS smart contract make sure that only the right boss can tell it to update itself to a new version?

A UUPS (Universal Upgradeable Proxy Standard) smart contract ensures only the right boss (owner) can update it to a new version through a combination of a proxy pattern, delegated calls, and robust access control mechanisms. In a UUPS setup, there are two main contracts: a proxy contract and an implementation contract. The proxy contract is a simple, immutable contract that holds the current address of the implementation contract and acts as the entry point for users. All calls to the proxy contract are delegated to the current implementation contract, meaning the business logic of the application resides in the implementation. The owner's address, which represents the "boss" with upgrade rights, is not stored in the implementation contract but in a specific, well-defined storage slot within the *proxy contract's own storage*. This is crucial because the proxy's storage is persistent and unchanging, even when the implementation contract is swapped for a new version. To update to a new version, a special function, typically named `upgradeTo(address newImplementation)`, is called on the proxy contract's address by the intended owner. When this call is made to the proxy, the proxy delegates the execution of the `upgradeTo` function to its current *implementation contract*. Within this `upgradeTo` function, or an internal function it invokes like `_authorizeUpgrade()`, a critical access control check is performed. This check verifies that the `msg.sender` (the address that initiated the transaction, which is the original owner) is indeed the authorized owner. Because the call is delegated, `msg.sender` remains the original caller, and when the implementation's code accesses storage variables like `_owner`, it is actually reading from the *proxy's storage context*. If the `msg.sender` matches the owner's address stored in the proxy, the `upgradeTo` function proceeds to update the internal pointer *within the proxy's storageto the `newImplementation` address. This effectively changes which logic contract the proxy delegates to for all future calls. If the `msg.sender` does not match the owner, the transaction is reverted, preventing any unauthorized party from upgrading the contract. This design ensures that the ownership control for upgrades remains securely with the proxy, guaranteeing that only the designated boss can initiate and approve contract upgrades.