Google Photos is a particularly successful app and service for the Mountain View, California-based company, offering users cloud storage and backups for their images. If you're willing to let Google compress your pictures a bit, storage is unlimited and your 'high quality' pictures don't count towards your cloud storage limit. However, if you have an iPhone, it seems that you can take advantage of a format loophole that gives you unlimited storage at 'original quality' resolution, which isn't the case with most other smartphones.
According to a Reddit post by user u/stephenvsawyer, the Apple iPhone series is able to take advantage of a loophole that lets users store unlimited photos at original resolution on Google Photos. Apple iPhone models can save photos in HEIC format, and Google's compression would actually increase the file size of the images.
Therefore, Google does not need to compress these images, and can directly back them up as they are. The images are therefore effectively being backed up at 'original quality' resolution, even if the user has opted for high-quality (compressed) backups. Notably, this doesn't apply to videos - which are still backed up at 1080p.
Interestingly, the new Google Pixel 4 series doesn't have this feature, and is the first in the Pixel series to do away with a feature that was one of the key selling points of earlier Pixel devices. While older devices continue to enjoy lifetime unlimited backups at original resolution, the facility hasn't been extended to buyers of the new Pixel smartphones.
The loophole for iPhones only works when saving images in the HEIC format, and users opting for JPG format images will have their pictures compressed when backing up to Google. Of course, users of all phones can back up images at original resolution, but these will count towards the Google storage limit for the account. Google currently offers 15GB of free cloud storage per account, to be used for all Google services. Users can, however, purchase additional storage by subscription.