The "NSP" file type and ".rar" archive format you mentioned are commonly used for updating the game on custom firmware or emulators like Yuzu .
: Files labeled as "NSP" distributed via ".rar" archives on third-party sites are often associated with software piracy. Downloading such files can expose your device to or lead to your Nintendo account being if used online. Official Sources Pokemon Legends Arceus -NSP--Update 1.1.1-.rar