Dropped phone in salt water pool - melted this bit, what is it?

@danp13762 that is your 5G mmWave antenna connector. I'd try and see if it can be resurrected. At least for the purposes you are thinking off.

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. Since iFixit does not yet have a guide for this phone, use these guides Samsung Galaxy S21 Ultra to help you disassemble your phone. Remove the main board and the charging board. I would replace the charging board and not try to clean it. It'll be better to replace it tehn trying to fix it. You will have to remove all the EMI shields from teh main board. If you do not remove the shields, you are not cleaning the board.

Take a close look at all components and connectors to assess the damage. Look for major corrosion, evaluate the pins in the connectors etc,. Take a look at all connectors as well as cable ends. One thing, salt water is very corrosive, so use some sterile water first, to get rid of as much of the residue as possible. Once you got any gross contamination cleaned, use 90%+ isopropyl alcohol and clean your parts some more. For a proper cleaning, use this guide. It was written for an Apple iPhone 3G but all the steps are 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.

The importance here is the cleaning. It would of course be best to get it ultrasonically cleaned but if you do not have it available, this cleaning will at least give you a chance. Do not forget to 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 (replace the battery) and re-evaluate. 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.