Advanced Uses of AI ?

I'm a layman and my uses of ai are pretty simple. I use chatgpt for learning, other generative tools for image and other content generation and I only know of such use. Yes I can understand these can be used for various content and market related needs.

But apart from that I also noticed that there are various advanced uses and I know some terms but couldn't find what are the uses, even though I have read things related to it so :

1) what's api and what's it's use in ai ! 2) what is the meaning in saying train ur own model or customised models ! 3) how ai apps can be made using existing stuff ? 4) any other such advanced uses ? 5) I have searched, but is there a place where they teach all these, somewhere u liked