Open Settings
Tap Sounds & Haptics
Under Ringer and Alerts, drag the volume slider to the right
Turn off Change with Buttons if you want the alarm volume to stay high
Make sure the alarm tone is not set to None
Open Clock
Tap Alarm
Tap Edit
Select your alarm
Tap Sound
Choose a louder alarm tone
Increase the volume while the alarm is sounding to test it
Check that Silent Mode is not affecting your alarm
Disable Sleep Focus or other Focus modes if needed
Restart your iPhone if the alarm still seems too quiet