The trick is to back up first, verify, then delete locally. On iPhone, turn on Settings > Photos > iCloud Photos and choose Optimize iPhone Storage, or plug into a computer and copy the DCIM folder to an external drive with a simple year-month folder structure. On Android, use Google Photos, tap your profile photo > Photos settings > Backup, choose Original or Storage saver, then Back up now over Wi‑Fi. Spot check 20 random photos and a couple of long videos in the backup, then note where your backup lives so you do not panic later.
Now free space safely using the built ins. On iPhone open Photos > Albums > Recently Deleted and empty it, and make sure Settings > Photos > Optimize iPhone Storage is on so the phone keeps small local copies while full res lives in iCloud. On Android, in Google Photos use Free up space to remove items already backed up from device storage. Clean messaging hoards: on iPhone go to Settings > General > iPhone Storage > Messages > Review Large Attachments to delete giant videos and GIFs, and on WhatsApp go to Settings > Storage and Data > Manage Storage to clear group dumps. Cull junk quickly by sorting to Videos in Photos, deleting obvious duds, collapsing Bursts to keep one best shot, and converting Live Photos to stills by tapping Live and choosing Off on older memories. Remove duplicates with Gemini Photos on iOS or the Clean suggestions in Files by Google on Android if you trust them, then clear Recently Deleted again and reboot to let storage recalc. Do this quarterly, and if you shoot lots of video consider converting old clips to H.265 with a simple converter to cut size without much quality loss.