How does this function retreive the variable "I"?

Quote:

How does the func(); function retreive the variable "i"?

With the "return" statement, which returns a value to the caller:

var funcs = [];
    
for (let i = 0; i < 10; i++) {
    funcs.push(function() { return i; });
}
    
funcs.forEach(function(func) {
    var i = func(); // note: you do not have to call it "i", you can give this variable any valid name
    console.log(i);
});

Quote:

When func() is run, does the code return to line 4, where the function was created?
Or is func() literally replaced with "console.log(a number)"?

You don't actually "return" to line 4: when you call funcs.push(...), you create a new function that returns a value, and that value is what i is. So, once you are out of your for loops, your array contains 10 function definitions: a function that returns 0, a function that returns 1 and so on. The .forEach iterates over all these functions one by one.

In your comment, you asked why it would always print "10" if you used var instead of let. The reason is scope[^] : let has block scope, var has function scope (or when defined outside a function, like here, global scope). So when you use var, then i will always be "the same i" because it has the same scope, which does not happen with let, because it has a different scope.