If you don't think the universe can intrinsically have its own state, but it needs to be stored elsewhere, then there are a couple of issues. If this memory can only have the properties of the universe and no more, and can't be observed, it might as well not exist. If it has other possible states, then it's circular, and you'll need some memory one level up to store the state of the memory. So if you believe in nested simulations, I suppose your questions could be answered up to the "root" universe.
Whether there is some "external" memory that actually stores all the state in the universe is at least for now pure metaphysics. Do you think anyone on Earth could possibly answer this question today?
If Bostrom's simulation argument is correct, then it stands to reason that some form of memory exists to store the "physical" representation. I find that people make the mistake of thinking you would need massive storage for this. You would only need some minimal representation of the world and the physical properties would be derived based on your observation. In other words, only render what is in your field of view. If you aren't looking at the atoms, then the atoms don't exist.
With the solipsistic simulation everything is fake, don’t worry about it, the input to the visual cortex simulation doesn’t even need to be what we think it is, because as a fake simulation of a mind, not even a brain just a mind… in a virtual box I’m being told all this exists by right down to the latest results at CERN it makes sense because the entire set of inputs is wired for me to make sense of this reality.
With a physical simulation however you can’t “forget” fundamental properties when simulating a universe of wave/particles. The atoms do “exist” when you aren’t looking at them otherwise how does the simulation know precisely what to show you when you look back? How does it know how those atoms are part of a ribosome, involved in an intricate quantum mechanical dance as they fold a protein and contribute to the growth of the plant on the shelf behind me.
Unless your suggesting somehow we’re in some weird middle ground when the simulation is full of shortcuts like we program into gamers today to save cpu or memory and somehow we haven’t noticed while probing the intricate workings of the universe via particle accelerators and other cutting edge experimental physics.