Adding fields to the post/page area

I know about Flutter and More Fields, but recently I’ve been learning how to manually add additional fields to the post/page areas. I’ve got that part down pretty good. But Flutter has a pretty neat feature where there’s a field for say, “Title” and you can have a link below that that says “Create another” and by pressing that it gives you another field to fill out.

I’d like to learn how to add that functionality to my fairly basic extra fields- creating additional groups of fields. I’m not sure how to search for that on Google. I was hoping someone could point me in the right direction!

Thanks!