https://wccftech.com/google-photos-gives-iphone-users-full-quality-unlimited-storage-for-free-pixel-4-left-out/

In an odd turn of events, iPhone users have found out that they are getting full quality unlimited storage in Google Photos for free. This applies to all iPhones and iPads, as they capture and store photos in HEIC format, which is already compressed and bypasses Google Photos' compression algorithm. It is ironic since Google's own new smartphone, Pixel 4, will not get unlimited storage in Google Photos at full quality, which was a hallmark feature of previous Pixel phones.

With the release of iOS 11, Apple moved from the old school JPEG format to HEIC (High-Efficiency Image File Format). As the format name suggests, this results in lower file size for photos, while keeping the same quality as before. Apple maintained support for the old JPEG format, by allowing users to switch to it as its compatible with a wider range of devices and platforms. However, the default photo capture format was switched to HEIC.