iPhone XR keeps restarting every time they unlock it

Ian ~ Does your iPhone XR still have its original battery?

And, if possible, what's the Maximum Capacity % here?...

Settings > Battery > Battery Health

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If your iOS or iPadOS device restarts or displays the Apple logo or a spinning gear unexpectedly

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Check your hardware

On an iPhone 6 or later, check your battery health under Settings > Battery > Battery Health.

If your battery doesn't need service, and your iPhone continues to frequently restart or turn off, contact Apple Support.

If your iPad or iPod touch continues to frequently restart or turn off, contact Apple Support.

If your apps quit unexpectedly, learn what to do.

If your device has a frozen screen, doesn't respond, or becomes stuck when you turn it on, learn what to do for iPhone, iPad, or iPod touch.