It's something I really should be taking notes on, myself. I want to make a sci-fi RPG akin to ME one day, and while I have the general details and races of my universe plotted out, I really should be trying to delve into the more specific details and historical events.
They had 5 writers doing Mass Effect. I almost dare say all 5 did equal amounts of work but I don't think that's accurate as some came on later, but there were roughly 4 of them for the duration of the projects who are responsible for each CritPath level and certain side-content.
Virmire is Mac Walters and Luke Kristjanson
Noveria is Chris L'Etoile's
Feros I don't actually know, maybe Luke, maybe Drew, maybe Mike Laidlaw who is also credited.
Eden Prime, Ilos and Citadel's story content is Drew Karpyshyn's
Patrick Weekes came on late and wrote side-missions, I forget which.
So that's the missions. The codex itself has details like planetary stats, which were initially created by Casey Hudson in an excel program he wrote, but afterwards Chris L'Etoile who had been assigned as their "IP Writer" due to his work on Asheron's Call would then go in and clean up the auto-generated facts to be more scientific, and he also wrote every single word in the ME1 Codex.
So that's an example of the team effort it takes to create Mass Effect. I don't think there would've been the same amount of historic detail with year and date if they didn't have a senior writer completely assigned to doing it. In Drew Karpyshyn's own spin-off novels he even skips ahead of some minutia by writing things like "It had been so long ago that the Captain lost track of the years." which I think shows that no one man can keep Mass Effect in their head.
That's why it arguably started falling apart from its integrity slightly in ME2 and then 3. all but two ME1 writers had left before ME3 and only one of them had stayed on Mass Effect since the beginning, which is Mac Walters so he was their "veteran" but he wasn't their Lore-guy despite writing excellent pieces like Bring Down The Sky, and he even admitted that he wasn't the guy who could come up with the "minutia". But with the Old Guard of Mass Effect largely replaced by the time they started doing DLC for ME2 it was people coming in with different perceptions and comprehension of Mass Effect, even if they all loved it.
I think that reading comprehension changes from person to person so it's because the original BioWare bubble was bursted that Mass Effect started to change, as well as the changes to the gaming industry and BioWare's role within EA.