Adding Captions and Comments to photos in macOS’ Photos.app is easy after you have shared it via iCloud photo sharing. But how to add titles or descriptions to photos you don’t plan on sharing right away, such as just cataloguing old pictures for easier searching?
The Photos UI is rater limited and no obvious place to add titles is visible. But select one or more photos, hit ⌘+I (or pick Info from the Window menu) and this opens a dialog with information about selected items, among other things, Title, Description, Keywords and Location. This is especially nice if you pick multiple items, you can assign title or description to all of them in group.
In the above examples I have picked two photos and assigned title, longer description and location to an old photo from 2001. All these fields are now searchable. I have used the description field to list people in the pictures, etc.