This is a part of Android natively. I assume that since most users intend to use Google Photos to manage their photos that it would request access to all photos.
I don't believe that Google would upload your photos remotely that haven't been backed up through Google Photos. Technically it sounds like their privacy policy would allow photos uploaded to Google to be used for training a model within Photos (e.g., I suspect their Ask Photos AI was probably trained on Google Photos data?) but it states that it won't be used for Ads or for training models outside of Photos.
I don't believe that Google would upload your photos remotely that haven't been backed up through Google Photos. Technically it sounds like their privacy policy would allow photos uploaded to Google to be used for training a model within Photos (e.g., I suspect their Ask Photos AI was probably trained on Google Photos data?) but it states that it won't be used for Ads or for training models outside of Photos.