What is going on?

I don't understand how anyone can be totally lost here. They use old records; social media, video feeds from security cams and peoples' phones, GPS data from those same phones, etc. to locate the exact time and place of a person's death. Or rather a time and position just before they died. Then they transmit the consciousness of someone from the future, overwriting that person's mind and imprinting the new one. Call it a tachyon stream - tachyons are theoretical particles that move faster than light and could be used to send a signal back in time. And how do they have so much information archived? Guess what, the NSA lied. They went ahead with their mass data mining program in secret. Other nations' intelligence agencies did the same thing. I'd be surprised if people in the future couldn't access quite a bit of info on just about anyone in this time (assuming they cared enough to bother).

Suppose you see a truck coming and swerve to the right, but the driver also swerves to your right causing a head on collision that kills you instantly. That's your moment of death. From the GPS in your OnStar system and the archived video from a traffic camera, the future knows your precise location and velocity during the thirty seconds prior to the crash. They beam a new personality, the Traveler, into your brain fifteen seconds before the fatal event. New timeline now. The Traveler swerves left and avoids the collision. And their growing organization has another agent.

They've studied up on everything known about you and your life, including job, family, friends, and so forth so they can impersonate you. There are things they won't know of course. People are likely to notice they've changed. But by keeping their mouth shut as much as possible in the beginning and letting others fill in the gaps, they can learn a surprising amount of that stuff without having to come right out and ask.

In the future humanity is in a gradual spiral to extinction. People are crammed into underground shelters designed for half the population they're supporting. But there's nowhere else to go. The Earth has been rendered uninhabitable by centuries of war that started with an asteroid impact in our near future. Food and other resource shortages ignited skirmishes between nations, enemies and allies alike, which escalated into World War III. By the time the dust cloud from the impact had settled it was in full swing. Antimatter explosives, along with nuclear and biological weapons, were used. Eventually designer pathogens that ended up killing off most animal life. It's too late for them to fix the damage they've done. The only solution is to undo it all, so it never happens in the first place. Understand? If you've seen The Terminator and weren't hopelessly confused by that, you get the basic idea.