/spawnpieces
This command spawns all nbt pieces it can find under a given path. The first arg is required. The rest are optional. You can make this command target only a single structure piece as well.
/spawnstructure
This command lets you spawn a structure from a given template pool. Only the position and resourcelocation arg is required. The rest are optional. (new update requires position arg unlike picture)
/spawnplacedfeature
Simulate worldgen for your placedfeature with this command. The biome check is ignored but the rest of the placements will be ran.