In Endgame, it seems that the Ancient One's explanation of
time-travel, and something I believe Hulk and Tony alluded to as well, is that going back in time does not change what has already happened, but merely creates a new, parallel universe, so that "time" is now shaped like a horizontal Y, that shoots off in two directions. This happened in the movie when Thanos travelled to the future and got snapped, since here now exists a timeline where Thanos no longer does his original snap.
this seems to create a paradox, because if we go back in time to the section of the time which is still linear, where does that time-line end up? In the newly created timeline or the original time-line?
See this drawing below (SPOILERS):
This is very similar to the imagery created by the Ancient One when talking to Hulk. The blue line represents time, which splits into two parallel lines. For example, the lower line could represent the original time-line where everything happens as we have seen it in the movies. The upper line could represent the time-line of the universe where Thanos, Nebula, and his entire army no longer exist, since they travelled to the future and got snapped. So, now imagine you are living at the time-point indicated by the red cross. Where does a person living in this time-line end up? The upper or lower time-line?