Insert Minecraft mod afterwards?

I started a Minecraft mod project with a few friends a few days ago. Now I've found a cool mod that I would like to add. This also includes important structures for the mod. Now my question: are these structures and ores generated even though the world was created before the mod was there?

No, the world is generated the first time you start…

Depending on how the mod is programmed, it may be that the newly generated chunks contain the structures or not…

Only in the new chunks after the mod is up, otherwise a new world has to come