In the associated solidity file, step4.sol, there are 2 contracts - ProxyContract and LogicContract.
To use this system, we first deploy LogicContract.
And then when we go to deploy ProxyContract, we pass the LogicContract's address as an argument in ProxyContract's constructor.
The ProxyContract is deployed only once.
The code of LogicContract will be called at the line 20. It will be forwarded with delegatecall while keeping the context of LogicContract.
In case we need to change the logic, we would deploy a new LogicContract and set the address of it with setLogicContractAddress setter function.
Note: The LogicContract we have here does not use the storage. Once you need to use the storage, the implementation becomes a bit more complicated because those contracts share the context.