upd.

Captain America finally shouting 'Avengers Assemble' (although he said assemble in a normal level tone voice) while the hero armies are gathered around him

Also Peter meets Tony during battle and rambles about how the last thing he remembers was he is in space, turns to dust, and next, he woke up and Tony is not there, and was told by Doctor Strange that it has been 5 years since and get ready to suit up.

does dr strange or thor have any cool scenes?
Dr Strange holding back a flood. Before, summoning all the ally armies via portals. Also a scene with and Stark where Tony ask if this battle is the 1 in 14 billion scenarios they win.

Could you please tell more about Bucky, or does he has literally no screen time? :)
Barely any screen time except a hug with Cap. You might see a brief fight scene but that's it.

and also going back in time: does this create some alternative realm or what? How does it work?
The time travel stuff is kind of confusing cos they reference lots of films such as BTTF, Terminator etc lol.
There was a scene where the Ancient One explains to Bruce that there are many dimensions and if the time stone is given to Bruce, it will create a split where the Dark Dimension will invade the main reality or something. I forgot what they said tbh