I wanted to know if there was a way to generate a structure other than a tree when a sapling grew, such as a villager house.

if it's your sapling block that there is no problem - because for the player, sapling is something that looks like a sapling and grows into something. so, in block ticks (random 1/minute ticks, do not make a ticking block entity), when it's time to grow to final stage, just pop the structure from the external file. the block need not be a sapling (in terms of extending vanilla blocks) at all.

6 hours ago, MFMods said:

if it's your sapling block that there is no problem - because for the player, sapling is something that looks like a sapling and grows into something. so, in block ticks (random 1/minute ticks, do not make a ticking block entity), when it's time to grow to final stage, just pop the structure from the external file. the block need not be a sapling (in terms of extending vanilla blocks) at all.

So how do I actually pop the structure?

examine the source code of this mod:
https://legacy.curseforge.com/minecraft/mc-mods/commandstructures

47 minutes ago, MFMods said:

examine the source code of this mod:
https://legacy.curseforge.com/minecraft/mc-mods/commandstructures

I've examined the source code of many minecraft mods and the game itself and nothing has made it any clearer on how to place a structure into a world. For this one, I look through the command class and can't tell what's doing the summoning

RawStructureSpawnCommand.java   rows 225-235  and 249-253