Great job on the explanation 👏🏽👏🏽👏🏽

I wanted to highlight a mistake on the initial code

instead of

pragma solidity ^0.8.7;

you have

pragma solidy ^0.8.7;

so I think it should be

// SPDX-License-Identifier: UNLICENSED
pragma solidity ^0.8.7;

contract HelloWorld {
    function sayHello() public pure returns (string memory) {
        return "Hello World!";
    }
}

I hope you found this helpful.