Here is a detailed breakdown of what this file is, what it does, and how it is used: Detailed Breakdown

Extract the POKLGEV...zip file to get the .nsp update file.

Use a title manager (like Tinfoil, DBI, or SX OS installer) to install the update over the base game.

Move the .nsp file to your Nintendo Switch SD card.

NSP files are typically used for homebrew-enabled or modified consoles (running SX OS or Atmosphere) to install updates from unofficial sources. How to Use (Context: Modified Switch)