Hi Christina_G12,
We appreciate you reaching out to us. We understand that your iPhone 11 powers on by itself after updating to iOS 15.
We’d like to gather additional details that may help us be in a better position to assist you.
Would you please tell us the steps you are taking to power off your device?
In the meantime, we recommend that you update your device to iOS 15.0.2 if you haven't.
While it isn't inherently assured that an update will resolve the issue, updating to the latest version of iOS for your device can help isolate potential software issues.
Kindly secure a backup and update.
How to back up your iPhone, iPad, and iPod touch
Update your iPhone, iPad, or iPod touch
Once updated, we recommend that you try the steps in the article below.
Turn iPhone off or on
Take care!