I use the Spotlight index/search feature of macOS to find documents of all types on my Mac. I'd like this feature for Joplin.
I noted today that when searching for a document that I had in Email, Evernote, Bear, and in Joplin, Spotlight located it everywhere except Joplin, which prompted this feature request.
I'm not savvy on how Spotlight works, but it probably indexes files and queries system facilities for the information it needs to build an index. From that I next assume Joplin would need to create/maintain indexable files that contain some/all the information in Joplin Notes, and maintain them.
That is about as close to a solution proposal that I can create with my limited knowledge of how Spotlight works. If would be extremely useful for Mac users.