The iPhone alarm uses the Ringtone and Alerts volume rather than the volume level used for music or videos.
- Open Settings > Sounds & Haptics.
- Drag the Ringtone and Alerts slider to the left to lower the alarm volume.
- Open the Clock app and confirm that the alarm still has an audible sound selected.
- Set a short test alarm before relying on the new level.
If Change with Buttons is enabled, the physical volume buttons can alter the alert level. Lowering it also affects other sounds tied to Ringtone and Alerts.
Before removing originals or changing an important setting, verify the result on the iPhone so you can correct the change while the original information is still available.