Q: What is the "other" category? And where can we find them?

"Other" includes everything that is not audio, movies, photos, apps, or backups. It includes downloads, work products such as documents, spreadsheets, presentations, Mail messages and their attachments, and OS X itself.

OS X: What is "other" space in About This Mac? - Apple Support

It is everything that does not fall in Apps, Photos, Audio, Movies, Backups categories.

Those could be, system related files, temporary, caches and so on. Many of those files are hidden and not visible in the Finder.

Some are inside System and Library folders. Most of them are hidden, as I wrote, and are located inside system folders under the root of your computer's hard drive.

If you are curious open Terminal and issue the following command

ls -la /