When using a map “with features” or a custom map created with the Tiles and the option “additional ores” enabled, you will find areas on the map where you find a lot more ores compared to the rest of the world.
This distribution is due to the real occurrence of ores on earth. You can find the source file here.
In addition to the vanilla ores, it is possible to put modded ores to the map. Some mods are pre-defined and can be selected in the Tiles software, but more mods can be added to the script by hand. For this case, QGIS already exports lots of ore distribution images for common and rare materials.
You can later import that images in worldpainter and assign custom underground pocket layers to that image.