If you made it past that thumbnail then you deserve to be spoiled.
Avengers Endgame and Time Travel
In the movie the gang goes on a Time Heist and they make a point to say “This is not like Back to the Future!”
It is exactly like Back to the Future with one key difference.
In both B2F and Endgame when they travel to the past they are either creating a new timeline, or entering the infinite multiverse version of events where that happened, your choice on which.
The Ancient One is mostly correct that if they can return the stone seamlessly after it is stolen there will be a very minor ripple on future events.
Boy, did they mess this up.
Even returning the time stone to The Ancient One seconds after taking it won’t preserve The Ancient One timeline as it is the same timeline where Loki escaped with the cosmic cube and Future Cap announced himself to Hydra and then left Past Cap to unknowingly deal with the fallout of this action.
Plan B of stealing it from even further back also isn’t seamless as Tony had potentially significant influence on his father which will likely change how that multiverses Tony is raised.
The reality stone can’t be returned to being inside Jane Foster… or can it? Maybe. It’s Thor 2 so who cares.
The power stone has to be returned to a universe where Thanos no longer exists, that Thanos came to our timeline.
The soul stone…. this is probably fine. We have no idea how it works. But it is also going back to the universe where Thanos is gone.
BUT WAIT, YOU SAID BACK TO THE FUTURE!
Yes, in B2F when Marty went back to 1955 he entered a multiverse where a future Marty (himself) used a time machine (multiverse machine) to travel to 1955.
He influenced that universe and at the end of the movie he traveled to that multiverses 1985. When Biff says “I thought you were up at the lake” That universes Marty is at the lake, there are two Marty’s now.
Back in universe 1, original Marty, as far as anyone knows vanished one night never to be seen again.
NOTE: The picture changing and Marty fading from existence until his parents kiss does not fit this model and I ignore it as it is dumb and would be a paradox if accepted.
What Endgame did different was they did not return to that pasts future, they instead kicked it into reverse traveled back through the multiverse to their origin.
Returning the stones, while noble and possibly necessary, will not fix the damage they wrought. They have to use the exact same timeways and must return to immediately after they left with the stones. Anything else and they are just entering a new multiverse.