Why wont my switch turn on after water damage and I tried fixing it

If your Switch is refusing to boot due to corrosion, then I believe that the motherboard is broken (along with some other components within the Switch). Sure the fan may come on, but that doesn't mean that some other component of the motherboard is still broken. So I would suggest a motherboard replacement, a guide on which can be found here (unless you can clean it somehow, which I talk about below): Sostituzione Scheda Madre Nintendo Switch

Other components of the Nintendo Switch may have also been damaged, such as the display or some other daughterboards (e.g. the MicroSD card reader), so I would recommend checking them for any corrosion, which can be cleaned up using this guide: come pulire correttamente la corrosione sulla scheda madre di un MacBook Pro (I know it is for a MacBook and not a switch, but the processes are basically the same for the Switch)

If you are unable to clean up the corrosion, you can simply purchase replacement parts here: Nintendo Game Console Ricambi

Do note that this sort of repair may be rather costly and take a lot of time, so if you do wish to carry out this I do wish you the best of luck with it.