I bought 2012 Nissan Maxima SV rbuilt from flood situation. The electric mirrors, windshield sprayer, seat memory, Seat heater don't work. What needs to be replaced to fix. Where do I start to find the problem and fix it.

Replace the car...

Or the entire electrical system... wiring and all. (its cheaper to replace the car)

Sorry but a flood damaged car will always be an electrical nightmare . THen it will have rust issues far earlier than they should and about every other part the water got into.