A family of Microsoft products that enable users to capture, organize, and reuse notes electronically.
These articles helped answer my questions.
I was using OneNote for Windows 10 (OneNote Store app), which was supposed to be stored in my OneDrive account, and that's why I couldn't find the local files.
The OneNote Diagnostics Tool didn't help, either, so I ended up manually (copy & paste) backing up all the content in OneNote for Windows 10, and switched to OneNote (formerly OneNote 2016). Now, OneNote is syncing across Windows, OneDrive, and Android.