: HD editions are usually "overwrites" or additions. You must already own the original game files (specifically pak0.pak ) for the HD mod to function; the zip file rarely contains the full game for legal reasons.
: Once inside the game, you often need to go to Video Settings and ensure the renderer is set to OpenGL 3.2 or Vulkan (depending on your source port) to see the high-resolution assets. Popular HD Projects
If your zip file is older or missing files, these are the current gold standards for Quake II visuals: