In our experience, only when you start with a new iPhone and migrate over, then your photo library is essentially empty of the “originals” and you get a very efficient storage of just the “optimized” versions, but then over time it starts to get filled up as you do things that need the originals.
Then as you take new photos and videos on your device, the new originals all remain on the device, and supposedly only when/if your iPhone starts “getting full”, does iOS try to offload the originals to iCloud to open up space, but in our testing, it can take a long time and we can never get our phone to recover much space, so we run into issues as we takes lots of new photos and specially videos.
We have videographers that take 100GB of new proRAW photos and videos in just one shoot. Their iPhones 1TB would then fill up in just a few days and so they had to resort to uploading them to Dropbox etc to open up space again and also to keep their phone’s photo library separate for their work stuff.
So now they can do a similar thing but instead with Photos+ and the fact that they can have unlimited pay-for-only-what-you-use S3 storage. It’s almost best to think of Photos+ as a Dropbox, OneDrive or GoogleDrive alternative but one that’s optimized for photos and videos.
We have many users that do use these other clouds configured with Photos+ instead of S3 and the main reason they tell us why they prefer to use Photos+ configured with GoogleDrive instead of just GoogleDrive directly is the “optimized photos experience” they get through Photos+ instead of a “generic file storage” experience they get with GoogleDrive. Photos+ is more like a fancy S3 client for them it seems.
Apple does scan photos server side not for image recognition, but for other purposes such as illegal content. Not that it’s a bad thing, but some people don’t want to accidentally get flagged for photos that aren’t illegal but that their AI accidentally flags as such.
One last issue with using 3rd party logins is that your account gets permanently linked to that specific email account. So you can’t change for example to a different google email later, that would require creating a new account, and if you lose access to that gmail account, you lose your login access. When signing up with just an email, you’re free to change emails when you like.