Ruben Paha

Fossils dont count as structures.
They are features like for example trees.

"features": [
    [],
    [
      "minecraft:lake_lava"
    ],
    [],
    [
      "minecraft:fossil",
      "minecraft:monster_room"
    ],
    [
      "minecraft:desert_well"
    ],
    [],
    [
      "minecraft:ore_dirt",
      "minecraft:ore_gravel",
      "minecraft:ore_granite",
      "minecraft:ore_diorite",
      "minecraft:ore_andesite",
      "minecraft:ore_coal",
      "minecraft:ore_iron",
      "minecraft:ore_gold",
      "minecraft:ore_redstone",
      "minecraft:ore_diamond",
      "minecraft:ore_lapis",
      "minecraft:disk_sand",
      "minecraft:disk_clay",
      "minecraft:disk_gravel"
    ],
    [],
    [
      "minecraft:flower_default",
      "minecraft:patch_grass_badlands",
      "minecraft:patch_dead_bush_2",
      "minecraft:brown_mushroom_normal",
      "minecraft:red_mushroom_normal",
      "minecraft:patch_sugar_cane_desert",
      "minecraft:patch_pumpkin",
      "minecraft:patch_cactus_desert",
      "minecraft:spring_water",
      "minecraft:spring_lava"
    ],
    [
      "minecraft:freeze_top_layer"
    ]
  ],

"minecraft:fossil" is actually listed in the desert biome just not at the top with all the structures
 

worldgen>configured_structure_feature>custom>tower.json

{
  "config": {
    "start_pool": "custom:tower",
    "size": 6
  },
  "name": "minecraft:village"
}

We can currently only use a vanilla structure as the "name": (in this case "minecraft:village")

Thats why his structure can only be referenced with "minecraft:village" and not with "custom:tower".

He has specificly chosen "minecraft:village" so he can use "start_pool": in the config.
"start_pool": allows him to reference a file in template_pool

worldgen>template_pool>custom>tower.json

{
  "fallback": "minecraft:empty",
  "name": "custom:tower",
  "elements": [
    {
      "weight": 1,
      "element": {
        "location": "custom:tower",
        "processors": "minecraft:mossify_20_percent",
        "projection": "rigid",
        "element_type": "minecraft:legacy_single_pool_element"
      }
    }
  ]
}

The "location": allows him to reference a structure he saved.

This method produces many problems like only being able to generate one kind of custome structure and of course also the one statet here
I am pretty sure this is not intended and feels more like a workaround.