“If God created everything, then who created God?”

There's a lot of places where one can poke holes into faith/the concept of a God, but I don't think this is one.

The reason being that God's existence doesn't actually change anything about the question or the answer. You can rephrase it as "If everything came from the Big Bang, what came before the Big Bang and what created the preconditions of the Big Bang?"

So you could use the same argument to "disprove" literally any world view, including science, or even hypothetical scenarios like the simulation theory ("If we live in a simulation, who is running the simulation?").

But you can not only "disprove" every potential answer to "where does everything come from", but you can also rephrase the question to "If atoms are made of quarks, what are quarks made of, and what are their components made of?" or to "If there's an end to the universe, what is outside of it?"

If you are smart enough though, you will see that none of that is actually disproving anything, because if you rephrase the question further it becomes "Why don't we know everything?" and that's a rather simple-minded question to ask. One befitting of a 7 or 8 year old, but not really of an adult.

Before the circumnavigation and the discovery and charting of all of the world, people also didn't know what was on the other side of the planet and still it would have been dumb to doubt what we knew (e.g. that the British Isles existed) only because there were large white spots on the map elsewhere.