What do you mean you don't understand these? You spent no time thinking about them? You have never used this sync feature? (Your mention of Google Photos folder makes me assume otherwise.)
I'm trying to understand what kind of internal/mental model of these concepts you had when you said you don't understand it.
Mostly because these seem trivial, but naturally these are still pretty complex features (if someone wants to program/specify/test/verify/replicate them).
So, I feel I understand them, but just because I have a very-very basic guess of how this might have been implemented. (Which is basically how I think I would have implemented it.)
To be honest it's a combination of "I should not have to care about this" with "I skimmed the documentation and Google's unclear warnings and they didn't clear my questions" and "I'm confused by Google's multiple overlapping products and their similar names". And no, I don't think I've ever synced a folder with Photos, the way I use Dropbox.
So here's a little history of my usage:
Once upon a time, I used Picasa. Can't remember if it was a separate product that Google bought or if it was always Google's. I stored pictures from my vacations there.
Then Picasa ceased to exist. I believe Google automigrated all the pictures to... Google Photo? Or whatever it was called.
Fast forward till recently, and I got an email warning that Google Photo was about to be end-of-life'd. Soonish. I wasn't sure if I had to do something about it. I followed some unclear tutorial by Google, and I think that created the "Google Photos" folder. I'm not sure. I'm also not sure if what I did was enough to backup my photos. I hope so!
Then I got my phone stolen! I thought I had lost all the pictures on my phone, some of them with sentimental value (I know, I know... backups!) Then I was surprised to find Google had automatically backed them up to... Google Photo? Google Drive? I'm not sure. I do know they are not easy to navigate now. I don't remember asking my Android phone to sync my pictures, either. Maybe I did. Who knows?
Nothing about this seems intuitive or trivial to me. I know if I put more effort into this I could understand it, but I honestly don't think I should have to. And I don't think regular non-tech users will find it understandable even if they read the documentation and tutorials.
Google themselves seem to understand it's not exactly clear, which is why they say "some users are confused by [...]".
Of course I think it's absolutely a mess. (Reading Google documentation is probably the most important thing NOT to do, it just confuses people while generally being completely unhelpful and less than useless.) And this is because Google is a master of failed communication. They just so suck at it, I don't know how can they consistently be so bad at it.
Anyway, the products and the concepts and the engineering and the flow is mind-boggingly simple.
Picasa evolved to Google Photos. The desktop client vanished. There's a Google Photos app, default on [all? most?] Android phones. And it has a backup/sync function, so if you connect your Google account it syncs photos to Google Photos. (This can be disabled, and there are probably countless apps that can/do backup to Google Drive though!)
Google Drive is completely separate, except they added an optional feature pseudo-folder. Which was bound to confuse people. Because [I guess] they exposed the underlying "files" from Google Photos. (Of course they are just blobs on a blobstore and tables in a DB.) So they connected that API/DB from Gphoto to Gdrive, and made it look like files. Which is stupid, because the semantics was never defined/cleared for users who are not somehow imaginary-systems engineers.
What do you mean you don't understand these? You spent no time thinking about them? You have never used this sync feature? (Your mention of Google Photos folder makes me assume otherwise.)
I'm trying to understand what kind of internal/mental model of these concepts you had when you said you don't understand it.
Mostly because these seem trivial, but naturally these are still pretty complex features (if someone wants to program/specify/test/verify/replicate them).
So, I feel I understand them, but just because I have a very-very basic guess of how this might have been implemented. (Which is basically how I think I would have implemented it.)