what is the reason for this, or do they work alone sometimes?
I've worked equally in both areas...however, most engineers will find themselves working in teams of engineers, each of whom is trained in a different specialty (ME, EE, Software, CE, etc).
Most engineers will find themselves working in teams of engineers, most of whom are trained in more-or-less the same specialty. If you are a chemical engineer working on chemical plants you most likely will work day-to-day with other chemEs. f you are an aeronautics engineer working on some aircraft you most likely will work day-to-day with other aeronautic engineers.
Most engineering projects these days are big projects. The work for any single discipline is much more than one person can handle. Large engineering projects tend to group people by discipline, with each group attacking the kinds problems they know how to solve. A well-designed engineering project will have narrow interfaces. A project that requires a large number of interdisciplinary teams is a sign of a bad design.
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