For a mechanical engineer, it is important to know that “design” as a process is not always a structured systematic close-ended analytical process as you have studied in your textbooks.
Mechanical design is a very Open-ended process and requires an open mind for the best results. There is never a single result, there can be multiple right results.
Mechanics, thermodynamics, fluid mechanics are highly structured methodologies to approach engineering problems, but design problems don’t always rely on these principles primarily. There are much more knowledge and skill involved apart from these subjects to make reliable, durable, safe products.
A high level of creativity is certainly involved in the process apart from engineering discipline and methods.