The easy to understand answer on how time travel works in general seems to be VERY similar to Back to the Future. Even if time travel follows the predestination paradox angle, it seems to follow similar [u]end effect rules[/u]as BtF. If you change the past and history will reflect it. Something Vex alterations don’t do. Simulated Saint is probably an exception to the rule due to the Vex studying him, his Ghost and his Light so thoroughly enough to manipulate it that he and his Ghost are able to use real Light outside of the Forest. Or the nature of the Sundial being powered by Wish Magic and Light gave him the ability to wield real Light. (Kinda like how wish magic used Light to bring Cayde back) That’s the short answer, here’s the dive. Younger Saint shows real time travel affects the present times history, so everyone only remembers the history after the change. When Marty changed his parent’s course in life, only he knew the change was made, his family had no clue. Elsie Brays Dark Future shows the traveler remembering the other histories and her success changes the true history but her lore doesn’t solve the mystery of the mechanics either. End results are then the only known yardsticks to use with our limited info. And the Vex manipulations do not have that same end results. Rather they appear to be making lateral moves in space time, transposing things from one timeline to another in a lateral shuffle within the normal flow of time. Because their alterations are not reflected in history like Saint was and we know for a fact that time travel was involved with him. That’s how they move an ancient structure into our timeline without altering history to reflect it existing here for all that time and why the Ishtar team remembered it not being there before. The Saving of Younger Saint is unclear in this aspect. Was he paradoxically saved by a predestined effect of a possible future, where he never truly died and thus the future didn’t know they even needed to save him? After all, we saved YS by mistake. We knew he met someone in the past that was mysterious but had no idea it was us even during the Dawn. Or is time more flexible? Did someone in the future go back and truly “rewrite” history and the present we know is the end result of that change? And we just stumbled into repeating it in our Perfected Timeline? Technically this is a crucial question for understanding mechanically how time travel works. We won’t know until it happens again or we learn about the Original Iteration of the Timeline. However the end result is the same; Changes to the past affect the history known in the present. So we don’t need to answer it for the moment. Instead we can take the example and then compare how the Vex actions stack up. And right away you’ll see the Vex Structures on Venus are different. When those structures appeared in Venus, history was not altered. The Ishtar team didn’t look at their logs and see they had been there all along. They didn’t suddenly have new memories. They said the structures registered as being old and native to the planet. As [i]if[/i] they had been there all along. Now you asked about Simulated Saint. Simulated Saint is “real” but as he was pulled from a sim, he isn’t “natural”. This was established in Dawn During the Dawn we had to stop a Cabal simulation from escaping with data it had gathered inside the Forest. The simulations are “real” because of the ontological power of the Infinite Forest, they are simulating reality under their desired conditions. And reality is real. It’s solid, flesh and blood. We were told when we went in that they are real enough to kill us. A magically super powered simulation of reality is therefore going to be “real”. Now as for Saint’s Light persisting outside of the simulation? I’d say that’s either a result of the in depth study they did with him and the machinations of taking it from it used a one of kind resource they can’t replicate. So this means Saint, his Ghost and his Light are very well calculated and known by the Vex. This puts him in the unique position of being so life like that he really can stand in [i]physically[/i] for the original. It’s also a real twisted manipulation of Osiris’ wish, in Ahamkara fashion. Or That’s likely where Wish Magic and our own added paracausal power (namely light ) to the machine as well. Much like how Riven’s Wish used Light to bring Cayde, it used Light to make the simulation “real” enough to replace the true Saint. He got to save the real one but got back a fake anyway, Ahamkara trickery too. Hope this clarifies the topic some.

It's not about time... Any given moment exists... We just enacted and finished a ritual that set the universe constantly looping in on itself... A stable point... Now, I just place if I want to... But they're just a tool... Like the guardian...