The easiest set-it-and-forget-it method is to turn on automatic cloud backup and let it run in the background. On iPhone, go to Settings > your name > iCloud > Photos and switch on Sync this iPhone, then choose Optimize iPhone Storage so it keeps full-res in iCloud but saves space on the phone.
On Android, open Google Photos, tap your profile photo > Photos settings > Backup, turn on Back up, set Upload size to Original if you have space or Storage saver if you want to use less, and set Use cellular data for backup to off. If you have Amazon Prime, the Amazon Photos app gives unlimited full‑resolution photo backup for no extra cost, so install it, open Settings > Uploads and enable Auto-Save for photos and leave videos off if you want to stay within the free 5 GB for videos. If you use Microsoft 365, the OneDrive app's Camera Upload is also solid and is under Me > Settings > Camera Upload.
In any of these, leave the app signed in, allow background refresh, and set uploads to Wi‑Fi only and while charging for a true hands-off setup. For real peace of mind, keep a second copy a few times a year on a computer or external drive. On a Mac with an iPhone, open the Photos app and click Import from your device, or use Image Capture to dump originals to a folder on an external drive. On Windows with an iPhone, plug in, tap Trust on the phone, then in the Photos app click Import or in File Explorer copy DCIM to a backup folder. On Android, connect via USB in File Transfer mode and copy the DCIM and Pictures folders to an external drive, then verify a few files open before you delete anything.
