So the Cro Route leads into the Miu route essentially.
Based on
this, MC got with a "girlfriend" and caused Chronos to appear to kill him, Cro sends him back in time to save him, which the only way to do that is to send him to before he got a girlfriend, and as a result, the world is in a state of quantum superposition, where everyone's possibilities are existing at once.
This is also were the infinitely long branch statement about space-time comes from, that if you can back in time and change the future, space-time must be infinitely long.
At the end of the Cro
route, it's again stated this all happened cause Cro tried to save Rei, every time Chronos attempted to reconnect with Cro, Cro uses Chronos power to save Rei, creating an infinite loop, which apparently results in separate timelines, the Rei we follow only experienced meeting Chronos 2x, but this same thing is implied to have happened a lot.
Then with the Miu
route, it pretty much confirms Miu was the girlfriend all along, it's the starting point, and Rei ends up back at that point in time? Don't really think i am getting everything there, as idk what would lead to Chronos initially needing to kill Rei, she wanted to kill Rei because Cro was saving him, essentially putting her in an infinite time loop