My minecraft "Versions" file doesn't contain Version I want?

If the Minecraft version you want is missing from your .minecraft/versions folder, the problem is likely that you haven't played that particular version before.

You can easily fix this by opening the Minecraft launcher, then starting the version of Minecraft that you want to get the textures from (in your case, 1.15.2). Once the title screen appears, you can close the game.

The files should now be present at .minecraft/versions/1.15.2. You may need to use a third-party program such as WinRAR or 7-Zip to access the textures, as they are contained with a .jar file that most operating systems cannot open on their own.