Question about adaptions

I noticed there are a lot of adaptions from novels on here and I was just wondering how that works?
Do artists contact the writer of the novel and ask if they can adapt it? Or do they write the novel with the intention of having it adapted later?
Just something I'd been wondering recently.
I've been on here for a few years but only recently become active, so I'm a little slow learning the ins n outs (esp with different terms).