Hi,
To get a crescendo feature for the alarm app, you will first have to set up Sleep, as the crescendo feature is only available with Sleep
To set up Sleep:
Go to the Clock app > Tap Sleep | Wake up > Then go through the process of setting it up in the health app.
After Sleep is set up, go back to the Clock app, in the alarm section at the top, it will have your sleep schedule, click Change > Scroll down until you find Alarm Options. From there click Sounds and Haptics.
You can then choose from the provided wake up alarms, all those alarms will start quiet, and slowly get louder to wake you up calmly. I hope I was helpful!