How do I fix my phone

Hi @ira40452

The phone has an IP68 rating which should make it waterproof for at least 30 minutes (at a depth of 1.5m) but since you said that the back has been coming off, it may no longer have this rating.

Check if the LDI (liquid damage indicator) is showing any signs of liquid having entered the phone.

If the LDI hasn't changed colour then it may be OK but maybe not. It may be safer to open the phone and do a visual check to see if all is OK first before turning it on. Here's an image showing what corrosion can look like in electronic devices.

If the LDI has changed colour then you will definitely have to open the phone and clean it inside as best you can.

The impurities in the liquid causes corrosion and provides circuit paths for the electricity which were not in the phone's operating design and could damage the components. The corrosion starts immediately and is ongoing until it is completely cleaned away

Do not try to charge or to turn on your phone and then remove the battery as soon as possible from the phone to minimize any further damage.

Then you need to dis-assemble the rest of the phone and clean all the affected parts using Isopropyl Alcohol 99%+ (available at electronics parts stores) to remove all traces of corrosion and liquid. If possible do not use "rubbing alcohol" as in some cases this is only 70% IPA or less, can contain additives which may leave conductive residues and is not as effective. If you do have to use it check the label to verify the amount of IPA. The higher the percentage of IPA the better

As always with electronics, especially surface mounted pcb be gentle when handling and especially when brushing away the corrosion. You do not want to remove any components from the board. Remove the shields that may be covering some components as the liquid may have got in under there. The ends of any flex cables and their connectors need to be cleaned as well

Hopefully after you have done all this the phone might possibly work correctly again.

Here is the ifixit Samsung Galaxy S20+ 5G repair guide that will help.

If this process seems too daunting, take your phone to a reputable, professional phone repair service, experienced in liquid damage repair and ask for a quote for a repair. If you decide to do this, do it sooner than later.