Hm ... I don't share your need to know the background behind all the powers involved in a story, so take my tips with a grain of salt 
Don't get me wrong, I love a good, satisfying reveal of how the powers work that makes you go 'ah, everything just clicked into place!'
But I'm not much of a fan of explanations for the sake of explanations, that serve no other purpose. So my favoured approach for creating (the lore behind) a power system is kind of similar to how you'd construct a murder mystery:
1) 'The Reveal'
For a murder mystery, that would be 'who was the culprit and why did they do it'; whereas for a power system this will be your 'ah, everything just clicked into place!' moment.
It could be the 'backstory' of how the power came to be (e.g. two gods fought and killed each other, and magic is their remains that got scattered to the winds), or the 'nature' of the power (e.g. it's the life force of your alternate universe selves), but you want this to be suitably epic/foundational and also clever/surprising for it to be a satisfying reveal. I wouldn't stress about it too much though; a good setup can make any reveal satisfying ...
2) 'The Clues'
For a murder mystery, the core circumstances behind the murder would dictate a lot of the clues left behind (location, murder weapon, who does/does not have an alibi etc).
Similarly, a core, foundational fact about your power system should carry some implications about how the powers actually work (e.g. if magic is the lifeforce of your alternate universe selves, maybe people whose birth was a very lucky coincidence would not have as much power as people whose birth was planned, anticipated and pretty much guaranteed). The nuts and bolts of what the powers can/can't do, what their costs are, etc.
I personally like to keep the foundational lore for an epic reveal, while revealing the more practical nuts and bolts of how the powers function as needed, when they are used in the story. Maybe you'd like to take a different approach on how you reveal stuff because your tastes differ from mine, but I still think it's good to have these practical facts about powers stem from a foundational 'theory' instead of being a laundry list of checkpoints, just so it feels more coherent 
Then again, this approach might not be as workable if you already have the plot established and you already know what your power system needs to do, but you don't know what your powers actually are. Nailing down 'what the powers are' first may lead you to discover unintended implications/side effects than can lead to plot holes, where (like Kaisei mentioned) the plot needs a character to not use their power (because the power wasn't originally intended to be applicable in that situation) but due to the nature of their powers and the implications that carries, it should be applicable.