Greetings, pdean, and welcome to Scrivener! For my recently completed bio, I actually stored all my notes (mostly written in plain old TextEdit) in the Finder, and then imported them into the appropriate scrivener project chapter as needed. I used about eight Finder tags for topics, but the primary tag was the date of the event, since my coauthor and I organized our bio chronologically.
The cards were stored in folders by year, with each decade getting an enclosing folder. Eventually we further enclosed the year folders in section folders ; the book was divided into about nine major sections, each covering a span of years ranging from about four years to seven years, IIRC.
Of course there were plenty of files / notecards that weren’t date specific, and those went into folders organized by topic. But most of my notes applied to a specific year or at least period (early 1940s, say) in my subject’s life.
I’m sure I could have organized the info more adeptly, but really, I found that if an item really needed to be in more than one place, it was just as easy to duplicate the card and file it twice or thrice in the appropriate folders.
Had I been storing all the info in a Scrivener project, I could have used features like cross references and keywords. I had good reasons for not doing that at the time, but they probably don’t apply to anyone else.
In the end though, I don’t think it much mattered, as chronology was really by far the most important organizing factor. I wound up going though the material so many times anyway that there was little risk of some precious tidbit getting overlooked. So my advice is, don’t freak out about the organizing system at this point! You can always add tags, keywords, references et al as the project develops.
For my bio, what really mattered was getting those all important dates down, and making sure I included appropriate reference info, which in my haste (limited archive access) I too often neglected, forcing me to to backtrack and try to hunt down original sources for a dozen cards or so because the original links had expired or I forgot to note a volume number. That mostly happened when I was new to this kind of research, years ago. Repairing those old omissions probably cost me the better part of a week – far more time than I spent organizing the info!
I hope this helps and am happy to offer any other helpful info, but as I noted, my project probably differs from most , for various reasons,including having a coauthor who worked in Word and not Scrivener, and my adopting Scrivener some years after the research commenced. I’d also be eager to hear about other biographers’ experiences. Good luck!