If what you download also contains title.keys, delete it. It is not needed. After downloading the keys, Open Ryujinx, go to the top left corner and click on "File" and then "Open Ryujinx folder". Open the system folder and put (only) the prod.keys file in that folder. Restart ...
but in the future, the Switch will be discountinued, and hardware does not last forever. As long you have a dump of the game, firmware and keys, you will be always able to play the game on PC and potentially other devices in the future, thanks to ...