If you have the fundamental belief that if we know every single possible detail down to the atom, that we can predict what will happen every time, then you believe that free will does not exist. If you think of it that way, and think everything is calculated, then it could be theoretically be possible for some kind of super computer to generate everything since it knows all the information and can calculate what will happen next.
I also think that free will may not exist, but I’m not sure why. Either everything can be calculated in advance as you say, or everything is fundamentally random because quantum mechanics. But maybe there’s something at the edge where neither of these explanations are correct? What happens there?
I don’t see any reason to think I’m not in a simulation, except that it’s just a silly ancient fable, created by the simulation. but none of that affects the “realities” of life and love all the rest of it.