Time travel is fictional. There's no actual rules to how it works, so any story you see it in will have different rules depending on what that writer believes would make for a more interesting story.

In that film, they're not using the idea that you can go back in time and change the future. Instead, they're saying that when you travel back in time you create an alternate timeline, a different future where events played out differently because of your actions in the past.

So in that story, it doesn't matter if you go back in time, you can't change the future you came from. You've just created a different future where a different version of you will live.

In the MCU, the way that time travel works is different than, say, Back to the Future. All time travel stories kind of make up their own rules.

When the Avengers time traveled, they basically just jumped into a different universe, at the time and place where they wanted to be. So they go from Universe A to Universe B, and then when they want to go home, they can bounce back to Universe A So nothing they did in that different universe would directly affect their current universe. If they went to some random person's house (John Smith) in Universe B and killed him, for example, that wouldn't affect Universe A John Smith at all.

So no matter how much trouble they got into in their mission, nothing they could do in any of those parallel worlds would affect Universe A. Except, as it turns out, that they managed to let the bad guys from another world come into Universe A, and that caused some issues.

Causal loop basically the reason for your travel back into the past no longer exists if you "cure" what happened in the past so there is no longer a reason to travel back to fix something, and if you don't fix it it stays the way it was.

The way he explains it is actually pretty good.

Replacing it with actual numbers:

You live in 2020.

You go back to 2000.

In that case, events in the past (2000 to 2020) caused you to do an action (go back in time).

Once you're back there, though, time still works the same way it always did. Events in the past can cause an effect in the future, but tomorrow isn't ever going to directly change today.

So, once you're back in 2000... the events of 2020, which to you are in the past (because you remember going back in time and everything that happened before that), can change your future actions (which to you take place in the year 2000). That means that if you can go back in time to change the past, you can change the past: your actions in your immediate future can cause changes.

But, the future can't change the past. If you went back to 2000 and did something there that made you not have to do that in 2020... well, you still did do that in 2020, because how else would you have been in 2000 changing things?

The idea is that once you go back in time, any changes you make are focused on the way you experience time, not some third-party objective time.

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He's suggesting (rather correctly) that time is relative. He's also suggesting that time is subjective (again, kinda correct).

Everything that you haven't done yet is in your future, no matter when in the rest of the flow of time that occurs. Everything that has already happened to you is in your past and therefore immutable for you. Is what He's trying to tell them. By going "back in time" they will be changing their present, not their own past.