For the rest of you, follow the guide above, install OptiFine 1.12.2, download a reputable pack, and watch the underground become an open book.
This article covers everything you need to know. First, let’s clarify a common misconception. A true X-Ray "mod" usually requires Forge or Fabric to inject code into the game. However, a texture pack (or resource pack) works differently. It does not alter the game’s code; it alters how the game renders existing blocks. x ray minecraft 1.12 2 texture pack
In standard Minecraft 1.12.2, stone, dirt, and deepslate all have textures that cover their entire surface. An X-Ray texture pack replaces these "opaque" textures with transparent or semi-transparent PNG files. For the rest of you, follow the guide