OK, so this film is a little confusing, right? I'll try to make sense of it here, particularly the ending - I'm sorry if this is a bit long. SPOILERS AHEAD.
The rules for how the past and present interact with each other are far more illogical in this film than in most time travel movies, which at first had me screaming "but that doesn't make any sense!" at my screen. But if we accept the unique rules this film plays by, it does at least follow them fairly consistently.
The most confusing rules seem to be:
1) The two leads' memories are not affected by timeline changes
2) The two timelines (1999 and 2019) are linked exactly 20 years apart in realtime, and the 2019 timeline is not affected until exactly 20 years after something happens in 1999. So for example, if on June 1, 1999, Yeong Sook decides at 6AM that she will kill someone later that day, and then at 10AM she actually does kill that person, the victim will not disappear from the 2019 timeline until June 1, 2019 at 10AM. Intent has no bearing on the future, only actions. Furthermore, since 2019 Seo Yeon's memory is not affected by changes to the timeline, for all intents and purposes it will be like the victim, despite being killed in 1999, does not actually disappear from the 2019 timeline until June 1 at 10AM in 2019. This is why she literally witnesses people disappearing before her eyes, and Seo Yeon will still have memories of that person from all the years between 1999 and 2019, despite them now not being alive during that time in the newly created timeline.
So at the end of the film, after 2019 Yeong Sook warns her younger self with a phone call, she somehow uses that knowledge to survive her fall from the second floor. How exactly foreknowledge of any sort would help making such a fall less fatal isn't clear, but regardless, it is revealed in the mid-credits scene that she has survived. Remember, at this point in 1999, Seo Yeon is still tied up on the second floor and her mother is unconscious.
Meanwhile, in 2019, Seo Yeon is safe, since 2019 Yeong Sook is no longer in the house trying to kill her. We assume at the time it is because Yeong Sook died falling in the past, but since this turns out to be false, we must assume that history has played out differently and there is some other reason that Yeong Sook is not in the house in 2019. At any rate, Yeong Sook immediately runs to the hospital to look for her mother, and not finding her there, runs to the cemetery, where she does find her mother, alive. IMPORTANT: This is because, as stated earlier, characters are linked exactly 20 years apart.
So let's say in 1999 Yeong Sook fell from the second floor at 1PM. Now let's suppose it took 3 hours for her to wake up. At this point, she proceeded to kill Seo Yeon's still unconscious mother (at 4PM.) This means that in 2019, Seo Yeon's mother will still be alive until 4PM, 20 years exactly after she is killed in the past. So after Seo Yeon is "saved" at 1PM, she has three hours to run to the hospital, then to the cemetery, and walk off with her mother at the start of the credits. But once those 3 hours have passed and it is 4PM, exactly 20 years to the minute past the time when Yeong Sook killed her mother, her mother would then disappear from the 2019 timeline, as we see in the mid-credits sequence.
Next, in 1999, after killing Seo Yeon's mother, Yeong Sook goes back upstairs, collects the still-bound child version of Seo Yeon, and locks her in the basement, where she keeps her for the next 20 years. Thus exactly 20 years later in 2019, Seo Yeon suddenly finds herself strapped to a chair in the basement in the film's final scene. Consistent with the way the movie has handled such changes to the timeline previously, her memories are not changed by the alteration of history, so she has no memories of how she got there or the previous 20 years she's spent locked in the basement.
Let me know if any of that was helpful to anyone, or if maybe you have a different interpretation of things. I'm sure there's stuff I missed, and honestly it's a lot to wrap my head around, so I'd be interested in hearing further ideas.
