Structures within the worldgen location are the definitions of the in-game structures. For instance, a village is defined as a structure, and can be placed as such. Structure definitions inform Minecraft about what the start (template) pool is, how high to place the structure (mostly on the world, but can be below ground as well), etc.

Minecraft wiki: Structure/JSON

Introduction

TODO

Using pool aliases

From minecraft snapshot 23w42a onward, Minecraft supports pool_aliases within a structure definition. This allows for changing the target pool in the jigsaw blocks used within the (NBT) structures to another pool.

One of the use cases is to create a generic set of roads, and use the pool_aliases to switch from a generic pool to a specific pool. For instance, a structure for a ruined badlands village (polygame:ruined_village_badlands) could switch from a generic roadset to a pool definition specific for ruined village badlands:

    "pool_aliases": [
        {
            "type": "direct",
            "alias": "polystruct:village/generic/roads"
            "target": "polystruct:village/badlands/01/ruin/roads"
        },
        {
            "type": "direct",
            "alias": "polystruct:village/generic/structs",
            "target": "polystruct:village/badlands/01/ruin/structs"
        }
    ]