understanding how google adwords headlines / descriptions work

It says I need five headlines. If I try to add less I get a "Too few elements in the collection" error. Does Google switch out between the headlines? What if I’d rather it just use one headline for all ads? I tried to copy / paste the same headline 5x and got an error about duplicates existing.

I looked at another existant campaign on my website and that campaign appears to have 3x duplicate headlines (not 5x). Is that campaign just kinda being grandfathered in?

And what if I wanted certain headlines to go with certain descriptions? From the UI there doesn’t appear to be a way to link the two.

Finally, does the order of the headlines / descriptions matter?

Any ideas?