“Storage full” usually comes from a few big culprits. Work through this list in order — the first four free the most space.

1. WhatsApp media: WhatsApp → Settings → Storage and data → Manage storage. Sort by size; delete large forwarded videos. Turn off auto-download of videos on mobile data.
2. Back up photos, then free space: in Google Photos, make sure backup is complete, then use Free up space to remove copies already backed up. (Check your Google storage quota first.)
3. Files by Google (or your phone’s file manager) → Clean: removes junk, duplicates and large files with previews before you delete.
4. Downloads folder: old PDFs, APKs and videos pile up here.
5. Clear app cache for heavy apps (Settings → Apps → [app] → Storage → Clear cache). Don’t “Clear data” unless you know you can log in again.
6. Uninstall apps you haven’t opened in 3 months — Play Store → Profile → Manage apps → sort by least used.
7. Offline maps and music: remove old downloaded areas and playlists.
8. Telegram and other chat caches: set cache limits in their settings.
9. Screen recordings and old screenshots.
10. Browser downloads and offline pages.
11. Move media to an SD card if your phone supports one.
12. Last resort: back up and factory reset — it also removes leftover system clutter.

More: back up phone photos properly and hidden Android features.