It sounds like you’re referencing a file named something like Mario Kart 8 Deluxe -NSP--CH--Update 3.0.3-.rar . That appears to be a (an NSP update for Mario Kart 8 Deluxe with Chinese language support, version 3.0.3, compressed in RAR format).
Update 3.0.3 (includes latest performance fixes and compatibility) Chinese (Simplified/Traditional) Compression: .RAR archive (requires WinRAR or 7-Zip to extract) Installation Notes: file to reveal the game and update files. Designed for use on Nintendo Switch consoles with custom firmware (CFW) or compatible like Yuzu or Ryujinx. Mario Kart 8 Deluxe -NSP--CH--Update 3.0.3-.rar
The components of the file name you mentioned indicate its origin and purpose: pirated or cracked Nintendo Switch game file It
The file refers to a compressed archive containing a specific software update for the Nintendo Switch version of Mario Kart 8 Deluxe 8 new tracks (Wii Daisy Circuit, Piranha Plant Cove, etc
Title: Mario Kart 8 Deluxe -NSP--CH--Update 3.0.3-.rar appears to be a compressed archive file whose name suggests it contains:
It is impossible to talk about .nsp files without addressing the elephant in the room: