Phone storage and iCloud storage are different. Your iPhone can be full even when iCloud has space, and iCloud can be full even when the phone has space.
1. Check the storage breakdown
Open Settings → General → iPhone Storage. Wait for the list to load. It shows recommendations and which apps, downloads and media use the most device space.
2. Offload apps you rarely use
Select an app and choose Offload App. This removes the app itself but keeps its documents and data, so you can reinstall it later. Delete App removes both the app and its related data.
3. Review large downloads
Look in Files, Podcasts, Music and streaming apps for downloaded episodes, films and playlists. Remove content you have finished. Check Messages for large attachments before deleting them.
4. Clean up photos carefully
Review duplicate photos, accidental screenshots and long videos first. After deleting items in Photos, empty the Recently Deleted album only when you are sure. It is a useful safety net while you decide.
5. Keep a little free space
iOS uses temporary space for updates and everyday tasks. Avoid filling the phone completely; having at least around 1 GB free can help it run more smoothly.