Call of Duty: Modern Warfare multiplayer fit into the narrative in a way that no other voice has. It apparently continues the story and involves characters like Ghost and Alex. Aren't those characters dead? How did Alex survive in Modern Warfare? And after all, how did Ghost survive too?
This is a real question. If you remember the ending of the Modern Warfare campaign, Alex (who you can unlock as a playable operator) apparently sacrificed himself to manually detonate bombs. This explosion probably would have killed most of the people.
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But of course not, and the game recognizes that too. When Ghost says "Alex, (I) thought you were dead", he simply responds with "Still standing". In essence, the hand of the game waves his noble "sacrifice" and he just says he got out without going into too much detail about how he survived. However, he is missing a leg and uses a prosthetic one. Maybe he will go into detail in a future Modern Warfare game.
He may have faked his death to start over as he left the CIA by joining Farah's forces in the campaign. Multiplayer now has it under the Warcom faction, which is a global faction fighting terrorism. In the Season 2 trailer, Ghost asked Captain Price to send fighters he could trust, and Alex showed up to help Ghost in the Season 3 trailer, showing that Price and Alex are still in touch in some way. He also indicates that Ghost and Alex have known each other since Alex calls Ghost by his real name, Simon. And Ghost knows who Alex is by looking at him.
This question is a little simpler. Ghost wasn't in the main 2019 Modern Warfare campaign. He was briefly teased at the end via a file from Price and showed up in first person in the season 2 trailer for multiplayer.
You may have thought he was dead and did, but not in this timeline. The recent remaster may have refreshed your memory when Ghost was burned to death in 2's Modern Warfare 2009, but those three original Modern Warfare games aren't canon in this new era. Infinity Ward restarted the series with the 2019 game, taking some of the same people and dropping them into a new universe. As this is a reboot, this Ghost is a new Ghost and he is therefore not dead.