If you see the code mentioned in this post on the top, that itself is a closure(the function b has access to var num that is outside the function b, which is also its lexical environment). What I meant by "combine all those functions (not one, but all)" is that, if you had many nested functions here like:

function a() {
    var num = 10;

    function b() {
        var newNum = num + 5;
        console.log(newNum)

        function c() {
            console.log(num)

            function d() {
                console.log(num)
            }
        }
    }
}

each of these functions has its own lexical environment, right? And this whole code is called closure[ combine all the lexical environments, you get a closure]. As it is closure, all these functions will have access to var num, and also any variables declared in the global scope.

I think that last line in my post is confusing. Will update it.