I don't believe in any Gods, can I be sure, im right? Certainly not... Don't know if you have seen the Netflix series "Love, death + robots"? if not its a series of small animations telling some short stories. One of them is called Ice age, which the image below is from, where a couple find a world inside their freezer and can see how it develop from early man into basically the space age. How do we know that we don't live in such universe, but we are simply unable to observe it.
The fact is that we don't and most likely we will never find out as we can't travel back in time.
But what we can do is try to look at what is most likely, based on what we can observe about the Universe in which we live. Ways we can try to judge whether we ought to believe one thing over another is to add things up and see how many things one ought to believe in to take a certain position.
So we could look at the something like us living in a simulated universe compared to a multiverse. Both offer no explanation to how everything got started, where does the multiverse come from or who made the simulation. So in that regard we can leave both as unknown. Then we can look at how the fine tuning of the Universe could explain why it is a simulation, but on the other hand we have no clue if these "values" could be any different, only that if they were, that things would go wrong. Also adding in the amount of energy to run basically an infinite amount of Universe simulations is very unlikely and also we would expect to observe bugs and shortcuts all over the place, which we don't. Just looking at a human, no programmer with a brain would design a "character" with such complexity that we observe, it would be a complete waste of time.
Obviously you can examine a lot more things about these ideas, but eventually you can add up all the things and at least to me, the simulation idea doesn't seem to cut it. And to me the best position to hold so far, is simply that we don't know. Obviously religion have their views as well, but again adding a God on top of it all and say he did it, is not going to solve anything, its basically just to push the question further out and just accept that God is outside time and space, without being able to proof it. Its no different than to just assume that the multiverse is possible, "because... it is"
But the biggest difference is, that those that suggest the multiverse idea, does not impose certain rules on people, either you agree with them or you don't, either way no harm done to anyone. And also those that proprose a multiverse have no issue admitting that its just an idea and nothing else.