Delete unused apps
Offload unused apps (Settings > General > iPhone Storage > Offload Unused Apps)
Review iPhone Storage (Settings > General > iPhone Storage) and delete large items
Delete large photos and videos
Remove duplicates in Photos (Albums > Duplicates)
Turn on iCloud Photos (Settings > [your name] > iCloud > Photos > iCloud Photos)
Optimize iPhone storage (Settings > [your name] > iCloud > Photos > Optimize iPhone Storage)
Delete downloads in Safari (Settings > Safari > Clear History and Website Data)
Clear app caches (Settings > General > iPhone Storage > select app > Delete App; reinstall if needed)
Remove offline content (Music/Podcasts/Spotify/Netflix/YouTube within each app)
Delete old messages (Messages > Edit > Delete All in a thread; also try Settings > Messages > Keep Messages > 30 Days)
Empty Recently Deleted (Photos > Albums > Recently Deleted > Delete All; and Files > Recently Deleted if applicable)
Delete large attachments in Messages (Messages > open conversation > tap contact name > Info > see attachments > delete)
Manage iCloud Drive storage (Settings > [your name] > iCloud > iCloud Drive > review and delete)
Delete old backups (Settings > [your name] > iCloud > Manage Account Storage > Backups)
Update iOS (Settings > General > Software Update)
Restart iPhone (press and hold buttons per model, then power off/on)