My iPhone 16 is useless! Help!!!

If the service centers did not reset the iPhone to factory settings and tested extentively before restoring from a backup, that would be the next thing to try. It's not impossible that there's a corruption in the software that restoring from a backup also just puts back. If they did that and the iPhone is still having problems, then it probably has a defect. Contact Apple Support or, if you have an Apple Store, take it to them for diagnosis and probable replacement.

Regards.

You will have to put your phone in DFU mode:

How to enter DFU mode on iPhone 15

In some rare cases, you may need to place your iPhone 15 into Device Firmware Upgrade (DFU) mode to reinstall iOS. Like recovery mode, you’ll need to connect your iPhone 15 to a computer using a USB-C to USB-C cable. If this is the first time connecting your iPhone to your Mac/PC (iTunes), you’ll need to go through the trust process explained in the recovery mode section above to establish a pairing with your Mac.

To enter DFU mode, do the following steps in quick succession:

Step 1: Quick-press Volume up and release.

Step 2: Quick-press Volume down and release.

Step 3: Long-press the Side button for 10 seconds.

Step 4: While long-pressing the Side button, long-press the Volume down button for 5 seconds.

Step 5: Release the Side button while continuing to long-press the Volume down button for an additional 10 seconds.

Unlike recovery mode, no connect-to-computer dialogue is displayed when entering DFU mode. After entering DFU mode, your iPhone 15 display will remain black. If the Apple logo appears, the process was not performed correctly.