Every photo a phone takes carries a block of notes: the make and model, the exact second, often a lens serial number, and — unless you turned it off — where you were standing, to within a few metres. Drop a photo in to see all of it.
Then strip it out. A JPEG or PNG is cleaned without re-encoding, so the copy you keep is pixel-for-pixel identical and simply smaller. The photo is read on your device and never uploaded; there is no server to upload it to.