In Infinity war we heard Dr.strange  saying to tony that he will not give his time stone to Thanos at any cost. and after defeated by Thanos, when he was going to kill ironman Dr.strange was willing to give his time stone to Thanos for Tony’s life. Now the question is why did he give the stone to Thanos As he said earlier he will not give the stone at any cost?

So, we can say he go to the future and saw they can win only once among 18……….CR battles. the reason is they can only win the battle if he gives the stone to Thanos. this is the 1st idea we can get from the Infinity War.

now talk about the death of heroes in the movie. actually, they are not dead. we know there are 6 stones- Soul stone, Time stone, Space stone, Mind stone, Reality stone, Power stone.

Soul Stone- Allows the user to steal, control, manipulate, and alter living and dead souls. The Soul Gem is also the gateway to an idyllic pocket universe. At full potential, the Soul Gem grants the user control over all life in the universe.
so, they just transported to another pocket universe and they are alive and will be back to earth in endgame.(spoiler- as we already know spiderman far from home trailer is out already)

now let’s talk about Avengers endgame final trailer: in the beginning, we saw tony is sending his last message to pepar as he thinks he will die in the space. then at the end of the trailer, he is preparing for the fight with Avengers. so, he came back to earth and sorted everything with Captain America. we can’t ignore ant-man. because he has the ability to transfer another universe as he has done that before.
on the other hand (as per comics) spider-man and Dr.strange will fight and try to get back in the earth with their brain and magic in the soul world. and yes Captain Marvel will help the Avengers to win this battle as She fights with aliens in many plants after leaving earth in 1995. we all already guess that everyone will back to life and ending will be happy.

this is the all knowldge i collected from comics and story line. hope it helps you.
if any point i missed. please share your thoughts in comments.
thank you.