minecraft mods

Got into some minecraft mods for the first time and was wondering how do they work.
Like if I download a mod which generates structures (ex: When Dungeons Arise) after generating the world, how does the world generation get affected? Does it simply spawn the new structures into unloaded chunks or does it have to be added before the world is generated?
Any help or info would be appreciated ;)

idk but i would not use the same world with different mods at different times. Just stick to one mod(s)/modpack with a world.

I think unloaded chunks but tbh you should probably start a new world from scratch to avoid corruption, crashes or other issues.

heard this one was pretty good https://jenny-mod.info/