Here is a definition of an Interior Designer:
Interior design is the total creative solution for a programmed interior. It encompasses the conceptual planning, aesthetic and technical solutions applied to achieve the desired result. A “programmed interior” means a specific intended purpose or use of the built environment.
What is Interior Design? | ASID Virginia Chapter
asidva.org/about-asid/what-is-interior-design/
An Interior Decorator does more of the planning of furniture, and color choices. You might use a decorator if you consulted someone from a furniture store, for instance.
I found this definition online for a furniture designer:
Furniture designers are the creative characters that develop design concepts and ideas for different kinds of furniture and home accessories.
Furniture Designer Job Description | AllAboutCareers
I want to respectfully mention that anyone can consult Google about stuff like this.