If the universe is a simulation, it is a pretty complex one
Ok, first off I'll say right up front this is all fanciful, more faith based rather than scientific. It's like debating which 'religion' is right.
But for the sake of entertainment, I'll discuss. How do we know the simulation is a pretty complex one? Comlpexity is a relative term.
So for one, there could be facets of a 'truer' reality we can't even conceive. Imagine Super Mario Brothers was a 'reality'. A third dimension would be an unthinkable exotic thing. In a very modern game, the concept of something having any smell or taste or touch are things that would not even occur to a hypothetical entity in one of those simulations. Similarly, we have quantized time and matter, meaning we effectively have a 'resolution' that may be coarse by a higher order existence standards.
For another, we don't know how much is *really* being simulated in this hypothetical. In a game, they don't simulate the other side of an object you are looking at. Similarly, there's nothing to prove that something that is not actively being observed truly exists at all times, rather than 'popping in' when observed.
Lastly, we don't know the depth to which things are really being simulated that we observe. For example, let's consider Half Life. To simulate that game, they didn't *really* have to model some weird physics thing that tears a hole between dimensions. However one could posit a being in such a world *believing* that a simulation would necessarily have to successfully model their *belief* in how things work. Many complex phenomenon and mechanisms could be in fact be glorified props, and the world written so that we believe/see in detail when it matters.
It's not a falsifiable claim, so it's not the realm of science, so we can debate this all day long with no real objective 'winner', but still it's fun...