Inside the .jar file of the version you want, find data/minecraft/structures - this is the list of all structures in the game. Choose a structure you want to modify, and grab yourself a structure block:
/give @s minecraft:structure_blockPut it in your world, change to LOAD mode and write the ID of the structure you want to change (e.g. if you have chosen data/minecraft/structures/village/plains/houses/plain_small_house_1.nbt, write minecraft:village/plains/houses/plain_small_house_1. Then hit Load to see where it would be loaded. You can edit the coordinates inside the structure block. Then click Load again, and the structure will be created. Edit it as you wish. The weird Jigsaw Blocks are blocks that spawn random structures nearby, they are better not to be touched (but you can change the block they will be replaced with by right clicking them). After you finished, change the structure block to SAVE mode and click Save. You can find the structure file inside <your_world_folder>/generated/structures/<...>/<structure>.nbt. Copy the NBT file into your data pack. For example, the file in the first example will be placed in <your_data_pack_folder>/data/minecraft/structures/villagae/plains/houses/plain_small_house_1.nbt. Now /reload and the structure should be changed. Travel to an unloaded chunk with the structure to see the change (chunks loaded before the /reload will remain with the old structure).