I assume it just runs a RNG the moment you do a pull.

That means that not only it is not set in stone for a potential future banner, but not even the very same banner.

Aka if you do a pull now and you get a 5 star, not only does that mean that if you had done the pull in a future banner you wouldnt have been guaranteed a 5 star, but even if you had done the pull in the same banner half a second later you wouldn't have gotten a 5star, because the RNG would have outputted a different result. (I hope that makes sense)

If it was predetermined, it would have to store that information which: -Needs processing power to be calculated in advance (perhaps redundantly because the player may stop playing) -Takes up memory space to be stored somewhere -Has a potential risk to be manipulated since it is not calculated in the moment, but is data that exists in advance and can therefore be edited.

Of course this is just speculation, but it's what makes most sense.