Spatula is right though, they do turn up out of nowhere with zero build-up.
Yeah, of course on a meta-level we all know that the heroes are going to show up after being dusted, they obviously are. But there is very little in way of actual build-up within the movie as to how they get there, who is going to be there, etc. All you get is, well, a scene of birds returning and Sam saying 'On your left' literally seconds before showing up.
As Spatula does in the post above, I'd also compare it to Lord of the Rings which is kinda the masterclass in these kind of 'everyone helps out climaxes'. In Return of the King you get very extensive scenes showing how the message for help gets to Rohan, you see the Rohan characters debate on whether to help, you see them ride out, gather forces, etc. We know they're coming, we understand their reasoning, we understand all the players in the field, and we know when the Gondor soldiers are on the losing end that help is on its way. And when it finally comes, it's this big climactic moment in the battle.
Meanwhile, in Endgame, you never see the other heroes until the Portal scene. We don't know how they know shit is going down at the Avengers complex, we don't know the wizards from Kamar-Taj were involved at all, Pepper Potts shows up as Rescue which nobody knew was even a thing she was doing, there's no lead up with our familiar characters getting undusted and realizing something is up, there's nothing. Not even an explanation how the joke "On your left" even worked, did Sam specify to unnamed Kamar-Taj wizard that he wanted to show up on the left of Cap? (just kidding with that final one, of course).
But like, imagine if Lord of the Rings was done that way. If you cut out all the scenes relating to Rohan, Aragorn, etc. and just have them appear at the end of the battle with massive Rohan and ghost dudes armies to save the day. That would be really weird. Now I don't think Endgame should've had extensive scenes of every hero coming to term with their return and deciding to help, but like, I dunno, one scene of Strange or Wong seeing everybody returning and using their powers to inform everyone about the battle would've helped.
It's an okay scene from a hype perspective, but from a narrative perspective it's incredibly dull writing. "And then everybody showed up to fight" isn't very interesting.
