At the end of the Resident Evil retribution movie, we see Wesker gathered what is left of humanity to fight the undead. But at the begining of the next movie, the battle is over and we only see Alice is alive. What happened in that battle? Which details are known about it?
Was looking through online and this was what happened. In the final chapter, they said that wesker had betrayed them. He most likely bombed up the place, leaving only him and Alice alive with all the zombies dead.
Another scenario is shown in the novelisation. Per the wikia
While The Final Chapter movie doesn't explain what happened at the White House beyond Wesker betraying everyone, the novelization shows the battle. While fighting the Undead army, its revealed that its not humanity's final stand but a battle between Wesker and Dania Cardoza, the head of Umbrella Research and Development who wishes to take his place. The Undead army is under Dania's control and she merges them into one giant monster called a Melange which consumes everything its path, killing Leon, Ada and Jill along the way. Alice kills Dania by telekinetically crashing her V-22, but the Melange kills everyone else in the area before dying. Wesker's injection contained an antidote that removes Alice's powers once she uses her telekinesis and Wesker leaves her to the Melange, but Alice manages to get into a bolt hole on the White House grounds and survive. Its later revealed that Becky survived as well in a safe room a staff member placed her within and is found at the end of the novel by Alice with the Red Queen's help.
I do not really know what actually happened as I haven't read the novel myself. Those were research online. Hope it helps