Guo Junchen explains his interpretation of the ending

Google translation: "The ending is that Yan Yue and Wei Zhi used Hua Rong together to restore the Three Realms to their original state, no one sacrificed, and returned to the best time, the two became mortals, Chen Yan and Wang Yue were lucky blessed to be together" 

credit to @Maria E  https://mydramalist.com/695281-hua-rong#comment-13517979

Some more speculation and explanations for the ending

https://mydramalist.com/695281-hua-rong#comment-13516571

https://mydramalist.com/695281-hua-rong#comment-13516571

https://mydramalist.com/695281-hua-rong#comment-13516679

https://mydramalist.com/695281-hua-rong#comment-13516349

I didn't want your useful information to get lost in the comments! It can be hard to scroll down and I'm sure many people had questions about the ending

Everyone feel free to add more theories