Kernel Panic - probably hardware issue

That's called boot-looping, or constantly restarting.

1/ It could be an issue with your software if this happened following an update, but Apple Store would have fixed it for you if it was.

2/ You could have a simple battery issue. In my opinion, Apple Store should also have diagnosed it and proposed to replace it for you. You could go back to them and ask them if it is. They should be able to tell.

If we take it for granted that the battery is not causing this:

3/ Charge the iPhone for 30 minutes. Then use the iFixit guide to disconnect the screen, then the battery (remove the screen, but only disconnect the battery. Don't remove it). Press and hold power button for 20 seconds. Reconnect battery then screen, then test the phone. Any change?

4/ Next, here's a way to go to diagnose whether the board itself is acting out, or one of the flat cables/components connected to the board:

Some of the components attached to the board could be causing this, such as the power button cable, or the charging dock, since this started when you plugged it into your computer. Try cleaning the charging dock using isopropyl alcohol (90% -99%) and a soft toothbrush. Next, you can use the iFixit guide to lift (or better disconnect the screen), disconnect the battery, the charging dock, and the power-volume-mute cable; then overlay a new/known good charging dock over the existing one, connect it in place of the old one, plug in the battery (should be known good battery, after Apple Store diagnose it for you), re-connect the screen, and finally plug a charger into the dock through a known good lightning cable. See if the iPhone restarts normally or changes behavior (when the Power/Mute/Volume cable is disconnected and you plug in a charger, it makes the iPhone restart).

If iPhone starts normally, try connecting the power-volume-mute cable, then the old charging dock. Test after each one. If anything brings back the issue, that's your culprit. Replace it.

If the phone still has the same issue after trying all of the above, then you would have established that it is some fault on the board. In that case, you'll need some good experience and good micro-soldering skills to establish the cause and fix it. This can happen following a harsh drop, liquid damage, an attempted repair, the use of a cheap/fake charger or charging cable, or sometimes just some components (there are hundreds on that iPhone board) kicking the bucket..

Good luck figuring this out. Those issues are usually a nightmare.