Smart Safe is a multi-signature vault. The entry point is SmartSafeProxyFactory.sol. It uses the EIP-1167 so that users only pay for deploying a Proxy contract that interacts with the Smart Safe implementation.
Your first deploy the src/core/SmartSafe.sol contract.
Second, you deploy the src/proxy/SmartSafeProxyFactory.sol and call deploySmartSafeProxy. This function will initialize a SmartSafe for the given _owners parameter.
From now on, the user will interact with the smart contract deployed by the deploySmartSafeProxy function.
Some features were inspired on Gnosis Safe source code.