If you are using a console with Custom Firmware (CFW) like Atmosphere: Move the extracted .nsp files to your SD card.
This specific file, , is an archive typically found on third-party sites containing the base game and its downloadable content (DLC) in a format used for Nintendo Switch emulators (like Ryujinx or Yuzu) or modded consoles. Super Smash Bros Ultimate (NSP)(91 DLC).rar