my phone is not booting up - apple logo is showing .. but not getting started

Have you tried the steps in the "If your device turns on but gets stuck during start up" section of:

If your iPhone, iPad, or iPod touch won't turn on or is frozen - Apple Support

Force restart the iPhone as follows:

• Press and quickly release the volume up button
• Press and quickly release the volume down button
• Press and hold the side button until the Apple logo appears

Since you tried everything, what did Apple Support tell you when you contacted them as suggested in the article above?