Looks like shiplap over drywall. Lovely product.

Thank you, "shiplap" is the word I needed to unlock a wealth of Google search results :)

From what I am reading it sounds like it might be reasonable to install shiplap directly over the studs. I wonder if one concern is whether the edges could be finished off cleanly in that scenario since the adjacent walls would be exposed, finished drywall.

Very Joanna Gaines trendy

What about ribbed walls instead? More on trend than shiplap walls (which of course means that it will be dated too at some point - just make sure that it goes with the house and you will still like it after it has gone out of favor). Use light-stained wood for a coastal look. https://www.pinterest.com/raygracie202/ribbed-walls-%2B-doors/