How do you make a mod using other mod stuff

I wondered if there was a way to make a mod using another mods things.

Example: Making a mod using AlltheModium and Ice and Fire to make AlltheModium dragon armor.

Note: this is literally just an example, what I want to know is how to import parts of a mod or if that is even needed at all.

I'm don't know much about coding and have no idea how to install APIs. (Or if APIs is how this is done.) So if anyone knows how to do this I would love to know.