It is a setting you can choose to enable when you install it. It judges “NSFW” images locally and automatically tags them as private, and you can choose to mark them "public" (as in they will appear in searches and albums and the like), they appear in your search. Or you can just choose not to enable that when you install it and it wont run that analysis. I have it turned on just in case something gets mixed in with my regular photos. I’d say 80% are false positives completely random pictures, or of bikini pictures or my gf laying on the couch in short shorts. But it does catch the occasional “private” image, so I like having it on. I also mark some photos private for various reasons, so I can pull up and show people my vacation photo album without worrying.
The review section is not for NSFW things. It is for low resolution or low information images. Again, it runs locally. As far as I know, it is to keep your library from being clogged up with crappy thumbnails or screenshots or other junk. If you don’t care, you can just bulk approve them.
It also does facial recognition locally. You aren’t giving up any privacy, which is why I use this after resisting Google photos and just doing the folders method for years.