Ending explained which I understood from my point of view...
Jin gyeom killed older version of him, then went back to the past again, since he has the ability to control the time, he goes to the day that he wants to alter and closes the time door, also leaves the SCARF of his mom as a MEMORY in that time line. Mom realizes what happened on future from it and hugs the high school Jin gyeom and thank him for it.
Since mom got the memory, so later at 2020 yoon tae Yi also remembers everything, because of same time line. Even the time door got closed Jin gyeom born during time travel, he still has the power, that he can only draw, but stays locked until he meets Yoon Tae Yi face to face.
This is what I understood no confusion...!
Jin gyeom killed older version of him, then went back to the past again, since he has the ability to control the time, he goes to the day that he wants to alter and closes the time door, also leaves the SCARF of his mom as a MEMORY in that time line. Mom realizes what happened on future from it and hugs the high school Jin gyeom and thank him for it.
Since mom got the memory, so later at 2020 yoon tae Yi also remembers everything, because of same time line. Even the time door got closed Jin gyeom born during time travel, he still has the power, that he can only draw, but stays locked until he meets Yoon Tae Yi face to face.
This is what I understood no confusion...!