Monoid homomorphism and isomorphism

I am reading the book 'Programming in Scala' (The red book).

In the chapter about Monoids, I understand what a Monoid homomorphism is, for example: The String Monoid M with concatenation and length function f preserves the monoid structure, and hence are homomorphic.

M.op(f(x), f(y)) == M.op(f(x) + f(y))
// "Lorem".length + "ipsum".length == ("Lorem" + "ipsum").length

Quoting the book (From memory, so correct me if I am wrong:

When this happens in both directions, it is named Monoid isomorphisim, that means that for monoids M, N, and functions f, g, f andThen g and g andThen f are the identity function. For example the String Monoid and List[Char] Monoid with concatenation are isomorphic.

But I can't see an actual example for seeing this, I can only think of f as the length function, but what happens with g?

Note: I have seen this question: What are isomorphism and homomorphisms.

To see the isomorphism between String and List[Char] we have toList: String -> List[Char] and mkString: List[Char] -> String.

length is a homomorphism from the String monoid to the monoid of natural numbers with addition.

A couple of examples of endo-homomorphism of the String monoid are toUpperCase and toLowerCase.

For lists, we have a lot of homomorphisms, many of which are just versions of fold.