1. 90,000 notes sounds more impressive than it looks.
This is his Zettelkasten, the German word for "slip-box". All notes were organized into 24 drawers and 3 filing trays (top left).
Not impressed? I don't blame you.
But the concept behind it might change your mind...
2/ His Zettelkasten's main purpose was to generate surprise.
He focused on organizing and constructing his notes, so he can "talk" to it. Every part of this process has its purpose. His notes are meant to lead him down paths he wasn't thinking about.