>...how do you add keywords to an existing file or folder? Like from Finder or the terminal.
As far as I'm aware, a folder can't have a Keyword assigned to it. Adding Keywords to files from the Terminal... I'm too Terminal-ignorant to help with that, but there should be a way. Many years ago — I've been on the Keyword bandwagon since before Tags were available — I found a way to bulk delete a Keyword from a bunch of files using a Terminal command. Since Keywords are often uncommon in most people's setups, it may help to search for other Spotlight-visible metadata that can be assigned to files from the Terminal, as it may then be a case of replacing that parameter with keyword in the command.
Most of my Keywords get assigned to files using Hazel. Hazel watches certain folders, looks for certain criteria, then runs Automator Workflows I've made which add the Keywords. If I need to more manually add Keywords, I have those same Automator Workflows in the form of Services, which are available from the the Finder Contextual Menu.
>That is the use case I am trying to do. I won’t ever be in a situation of saving a file with a keyword. The entire idea of my automation is that I don’t manage these tags/keywords myself.
Sorry to sound like a product shill, but that's why I rely on Hazel. It does the bulk of the work for me assigning Keywords. Just to describe what that means, let's say I get PDF invoices from five vendors. They go to the Downloads folder that I've set Hazel to monitor, Hazel looks for specific content in the PDFs (usually something like an account number or remittance address), then runs the specific Automator Workflows assigning the Keywords I want on each invoice, and finally Hazel moves the PDFs where I want them.
>I guess the biggest [keyword value] is likely since it is a common and standard metadata that it works across platforms and not just within the Apple ecosystem?
Agreed. And to me the greatest value in that isn't in sharing files with others — what are the odds a Windows user will care what Keyword I've assigned to a file? — it's that the copies of my files that are backed up to cloud services retain the Keywords. Second to that, the biggest value for me in Keywords over Tags is that I can easily add as many as I want.
>You mention TextEdit, and Word. In both cases when I go to save a file since it uses the Mac native file save UI, the second box is tags.
In TextEdit, File > Show Properties is where Keywords are added (to RTFs, etc., but not .txt files).
>You are right that in Word if I go to properties, I see Keywords and not tags. It is not clear to me if this is a limitation of Tags or just an oversight in the application.
I believe that's an app limitation, but I'm not running anything near the latest macOS or Word.
>I was curious so I took an image, added a tag and opened it in preview. And the inspector panel does not show the tag.
But the Magnifying Glass section of the Inspector Panel will accept Keywords. Tags, as far as I'm aware, are meant for Finder and may largely (completely?) be limited to places like that (aka, the iOS Files app also), but not for within apps.
>So it seems like a bit of a tradeoff, with keywords you get metadata on the file itself that is very portable to any system. But being able to view the tags is limited to applications.
Just like Tags, you can create Smart Folders in the Finder for Keywords. Or a Smart Folder for files with Tag:Red and Keyword:InProgress. That kind of flexibility is definitely a great feature of Smart Folders.
I hope this helps. I'm not out to convince you to switch from what sounds like a thorough, effective setup on your end. But I do hope this helps in any way.