in The Problem with Callbacks: Callback Hell lesson

I just want to know the retriever string in dog.txt how it's working with

superagent.get(https://dog.ceo/api/breed/${data}/images/random).end((err, res) => {
console.log(res.body.message);
});

when I play with retriever string by change some letters then it will not working !!
the retriever it's the results of data that's clear
but the retriever it self from where it's coming ?