The first is the limitations... That is, history cannot be changed

Say:

1. Zhu Yue's body was killed in the siege, so Zhu Yue naturally had no body in the following time.

2. After the protagonist goes back to save Zhu Yue, he separates Zhu Yue's body and spirit, and only brings his body back to modern times

3. After returning to modern times, Zhu Yue can be attached to the physical body, but in the previous history, Zhu Yue's physical body still did not exist

Conclusion 1: History has not changed, but it has saved the parties

Conclusion 2: You can only modify the past on a very small scale, and you can't do anything about big events

The second is the theory of cause and effect

It was precisely because the body was destroyed during the siege, and Zhu Yue didn't know Emiya Shirou, there was no other way. That's why she stayed in that gap and waited for Bai Ji's fall---> it was because Zhu Yue was waiting for Bai Ji's fall that Shirou would intervene---> it was because she intervened that she would challenge to return to the past---> it was precisely because she returned to the past that the law of time would be exposed, and the jewel Weng would also record the existence of the fourth law

Conclusion 3: In short, it is the interweaving of cause and effect, which is also borrowed from the reverse cause and effect of a certain book, after all, the ideas came at the same time

That's probably how it feels