My motherboard is dead. Need to replace it.

@mdzobaerahmed the bottom line here is, that you may have to replace the motherboard anyway, I would suggest you at least give this a try and see if you can resurrect your phone.

For now, you must stop continuing to try to charge, sync or otherwise use your phone. This potentially can make a bad situation worse. This may further damage your phone.

The very first step I would take is to disassemble your phone. Use these guides OnePlus 7T to help you disassemble it. Take a close look at all components and connectors. Truly assess the damage. Look for any major corrosion, evaluate the pins in the connectors etc,. Take a look at all connectors as well as cable ends. You will have to remove all the EMI shields from the motherboard. If you do not remove the shields, you are not cleaning the board. Once you got the gross contamination cleaned, use 96%+ isopropyl alcohol and clean your parts some more. If you have any questions about any part, you can always post some pictures of it with your Question. That way we can see what you see.

For a proper cleaning, use this guide. It was written for an Apple iPhone 3G but it is still pertinent to your phone as well. I can not stress the importance of a good cleaning enough, so do it over and over while replacing the alcohol after each cleaning. Do not use compressed air for the cleaning since that can drive liquid as well as corrosion particles into areas that will cause trouble later on. You do not need to leave the phone to dry for a day. Isopropyl alcohol in the higher concentration will evaporate quickly and dissipate the water.

The importance here is the cleaning. It would of course be best to get it professionally cleaned with an ultrasonic cleaner. After that you must replace the battery. This is a must and not an option. It will fail if you do not take care of it now. After that, reassemble your phone and re-evaluate for any further damage.

For as long as you have not cleaned your board and replaced the battery, everything is just a guess. Keep in mind that water damage is the hardest to troubleshoot and to repair. It does not matter if is was exposed for 2 seconds or 2 hours, what matters is where the water (particulates) did the damage.

If you still need to replace the main board, check places like this and many others, for a replacement