Here’s an interesting twist of fate. Google Photos used to offer free unlimited original quality photo backup for the Pixel phones as a perk of owning one of the devices. This courtesy, apparently, hasn’t been extended to the new Pixel 4 and Pixel 4XL phones. You’ll only be able to save compressed jpg backups to Google Photos on the free unlimited account.
It seems, however, that due to the file sizes offered by Apple’s HEIC/HEVC encoding, Google Photos doesn’t need to compress the files to optimise them for storage. In fact, recompressing them would result in a larger file. So, the original image quality captured by the iPhone is retained with the free unlimited storage on Google Photos.