Okay I think I figured it out. At least some of it. I think both MC and Jinwoon were reborn. Because Jinwoon also hears the ringing when the past is changed just not as loudly. You can see it in the red ring as opposed to the blue. Plus he just kissed his hand and that is something he shouldn't have known. Plus he has dropped hints that he remembers more than he should at that time. I think since we didn't see who got hit by the truck at the beginning the second time. I feel Jinwoon chased the MC and died with him but MC doesn't know that.....this is just a theory I do not know if any of it is facts or not.