I don't really get how you can merge a timeline

So splitting the timeline? Makes sense. Downfall timeline? Messy, but makes sense as a what if.

Merging the timeline? Just doesn't really make any sense to me, time travel isn't realistic, but it does have a logic to it that changing an event makes an alternate timeline.

I don't really understand how two of those timelines could ever merge, unless they somehow naturally became identical to each other. But that still isn't really a merge, as stuff from one timeline shouldn't be able to affect another.

I don't understand how it could even happen because there are two different universes with their own locations and people. There's no "open space" to put the timelines into, as they exist in the same point in the universe.