NSP files are used by the Nintendo Switch to distribute and install games. These files are essentially containers that include all the data needed for a game, similar to how ZIP files work on computers. However, NSP files are encrypted and can only be decrypted and used by the Nintendo Switch, or through emulation with specific software.
If you love the demo, the next step is to buy the game. The "Star Ocean the Second Story R Switch NSP" is simply the digital version of the game you can purchase and legally download directly from Nintendo.