Joseph L. answered 23d
Coding, Game Dev, AI and Tech Solutions Professional
Designers and developers can, and sometimes are the same individual, specifically on small teams or projects.
If they are treated as two separate roles then design can focus on gameplay, feel, monetization strategy, visual or the general direction of the game. These are often experience or UX designers, artists or producers that set the goalpost for what is expected and acceptable from the technical implementation.
Developers often refers to the individuals doing the work-on-the-ground. Implementing the designers' vision through graphics, code and tools and are usually technical artists, coders or engineers. It can also refer to the team or company in general as in: "The developer of Mortal Kombat is Netherrealm Studio."
If everyone is doing their job properly than good design and development become hard to distinguish and the two roles complement each other. If someone messes up their impact usually becomes more exposed as players are more likely to clock something they don't like. In most cases, cross-function communication is key to success in any role.