apple logo appears after switching on but it goes on and off
Why does my iPhone X keep turning on and off the Apple logo. Battery is ok, tried all steps described in apples support site
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It might be that the phone is stuck on a boot loop, but it's hard to tell with limited info about what happened before the issue began showing up.
You can try to boot into recovery mode and use a Mac to update iOS as detailed in the If you can't update or restore your iPhone or iPod touch - Apple Support article. The problem may be fixed by updating your iPhone, which preserves your settings and content. However, if updating doesn't work, it may be necessary to restore the device, which erases all settings and content. If you backed up your iPhone to the Mac you'll be using or iCloud, you'll be prompted to restore the backup onto the iPhone.
Hope you get your iPhone up and running soon!