For the express pass timed entries, those are chosen at the time you get your express pass. You pick from a pre-selected list- at least, if you buy them direct from USJ. I have heard that you’re randomly assigned them through some third party sites.
For Super Nintendo World for the non-express pass timed entries, they gave you options for the next available two hour range. It’s not like you could go in the morning at 9 am and pick a time for 4 pm- it’s a moving target, they only offered returns around noon at that time, because that was what was next available. (I think we picked our return time of 6 pm in the late morning, but I don’t quite recall right now.)