Maci Fudge the turning off and then the "it showed the Apple logo but it started flickering green and black" sounds like water damage to the logic board.
For now, you want to stop continuing to try to charge, sync or otherwise use your phone. This potentially can make a bad situation worse and may further damage your phone.
The next thing to do is to clean it. Disassemble your phone using iPhone XS Max Repair these guides . You must remove all EMI shields. If you do not remove those, you might as well not do anything. After you removed those, clean the whole board, connectors and all with +90% isopropyl alcohol. Follow this guide , and even so it was written for a iPhone 3G, all the points are still pertinent to your phone. While you clean your board, check for any obvious damage, like burned or missing components etc. Do not forget to clean the connector and the flex cable ends. When it is properly cleaned, replace the battery. All these steps are to avoid delayed failure caused by corrosion. Best thing to do would be to clean it with an ultrasonic cleaner, but if you do not have access to one, this will at least help. Once all this is done, reassemble your phone and reevaluate. Until it is cleaned, everything will only be a guess. Even with all this, water damage has only a small chance to be fully repaired, but at least you can give it a try. It is possible that yours is just the display that is shortened out. Consider changing it after you clean the water damage.