Go to School
Saying the lore in Starfield is expansive is the understatement of the universe (pun intended). Like most of the things in Starfield, you are given a few references and ideas about the collapse of Earth, but you’ll need to literally go to school in order to find out the specifics.
The first couple of missions will see you head over to the UC Vanguard headquarters in New Atlantis. If you choose to join them (and Yes, you should join every faction in the game), you will be told you have to go downstairs and watch the history of New Atlantis.
This will take you about 10 minutes in real time, and have you listen to pre-recorded messages, pop-up screens and LOTS of info regarding Earth, the collapse of its magnetic poles and everything leading up to the present events of Starfield. It’s a must have for anyone interested in the story of the game.