How do sales pages and print ad are completed?

I have made a lot of writing work and I want to use it as portfolio to get copywriting gigs. Some aspects of this art confuse me especially sales pages and print ad. These two very important parts of copywriting involve a lot of design which is really confusing.

How does it work usually? do you get the skeleton from the designer and write to it or do you write whatever and then you give it to the designer?

It wouldn't make to write a sales page before you get the skeleton, otherwise what do you even write? it wouldn't be much different than a regular copy