Since my days as a PhD student, I have worked on a general theory of the design process. In my Ph.D. dissertation, I developed the first draft of a theory/model. I have used this model since those days. It has evolved, but the core model has been the same. I still use it when I teach and even more when I think about design, designers, and how they work and think.
Nowadays, I call it a configurational theory since it only captures elements and relationships. It is not prescriptive. It is descriptive. As a configurational theory, it is meant to be used as a foundation upon which other aspects, ideas, and theories about design can be applied or layered. The present version is found below.
I will not here go into any details of the theory, just give some hints.
The thin red line: Everything about the thin red line takes place in a designer's mind (including everyone involved in the process). Everything below the red line is manifested in the world in some form (text, visuals, objects, etc).
The design situation: All design work takes place and ends up in the "design situation".
Thought figures, ideals, values, repertoire: This is what is in the mind of a designer that influences their creative thinking, problem-solving, choices, and decisions.
Vision: This is what emerges when a designer's mind collides with a design situation. It can be a vague notion of where to go, what direction the design work should move, or what would be some preferred qualities of a final outcome.
Operative Image: The operative image is a consequence of the creative and critical work of the designer when it moves from a vision toward something more concrete, such as sketches, drawings, statements, prototypes, etc. Part of the operative image is manifested in the world but part of it is still only in the mind of the designer.
Final design: As the name says, this is the outcome of the design process.
Arrows: All arrows show how elements are influencing each other, and all arrows go both ways.
Flow and time: There is no defined way that the process flows through the model. It can start anywhere. It can move in any direction. It can be fast or slow.
All elements and all relationships are present in every design process.
This model can be used to analyze any design process. It can help explain how certain design processes move and flow. It can help understand where issues and problems might be in a particular design process, but it does not prescribe how to do design in any detail.
To me, this model has proven to be exceptionally useful over the years. Of course, to explain the full richness of the model would take up much more space and does not fit here. Let me know if you want to know more.