Quake II RTX is not a game, it's a new renderer and texture pack for the original Quake II.
Think of it as a mod, if you want. You can't play a mod without the game it's made for, and in this case the game is 1997's QuakeII for Windows 95. What the RTX mod does is update the front-end to run better on modern PCs, use a new path-tracing renderer, and update the textures to include information needed for path tracing that the original never had, such as reflectivity, emissivity, and roughness maps.
The reason Quake II RTX is in the discussion is because that is a free addon for the 1997 game, originally distributed on CD-ROM only, and also compatible with Steam/GOG digital storefront versions that were reworked a decade later to play nice with modern OSes. This new 2023 remaster is only available to anyone who bought a digital storefront version. You're not entitled to it, or the MachineGames DLC content, unless you buy one of the digital storefront versions, which is great for anyone who bought it digitally over a decade after Quake II was mainstream, but a middle finger to the majority of the audience who have, to date, always been able to do everything with their original purchase of the 1997 version.
