Let's start with how time travel generally works in fiction.
Fixed Timeline: You successfully kill adolescent Hitler, but a new Hitler figure, regardless of how or explanation, starts the Holocaust and WWII still happens. You still exist because no matter what you do in the past, the future must adhere to it. This is essentially called a closed Time Loop. The future is always the same because you were ALWAYS MEANT to change the past. It means the original Hitler was always killed by you.
Dynamic Timeline: You kill adolescent Hitler, and world does not experience WWII related events. Your Grandparents never meet. You shouldn't exist in the present. If you return to the present, this is what creates a PARADOX, depending on how people interpret it. Either you disappear from existence or you exist but should never have existed at all from this point on.
Alternate Timeline/Multiverse: a slight combination of the above. You kill Hitler in the past, but YOUR timeline does not change, it still happens. The idea is a new timeline is created when past events are altered, hence there is now a new timeline where WWII did not happen and you were never born. You didnt erase the events from your own timeline, you essentially created a new universe.
Now think about what happens in Endgame as the introduction of the Multiverse. Captain America went back in the exact points where they retrieved the Stones plus Mjolnir, THEN he just created another universe in which he gets to live his life with Peggy. In this timeline, Cap could easily have stopped Hydra, saved Bucky, saved Howard and Maria, etc, BUT whatever happened in the original timeline didn't change, as Professor Hulk explained he lived his future in a different past, that's why he returned in his original universe as an old man to give Sam the shield.
At least that's how I see it lol