What is a more introverted job software or mechanical engineering?

Software engineering is also collaborative. (1) You develop a specification, which must be reviewed and approved by a committee. There are at least two iterations of formal review. (2) You write the code, and then it is subjected to a peer review; and there may be more than one iteration of this. (3) Your code is subjected to testing by someone else; and there will be a formal process of discussing and correcting any errors found. (4) When it is released to the user community, you will be called on to discuss and correct any problems that appear. Thus, I would argue that software development is not quite an introverted activity. BTW -- The amount of formality and bureaucracy you will find in this will certainly depend on what kind of organization you work for. In a small shop with only one or two programmers (tweaking their inventory or billing software) most of the review process will not exist, because management is unaware of the discipline and staff to do it is lacking. In a large organization with a complex product (operating system, for example) and management with software development background you will find all the checks and balances in use.

In terms of project opportunities, software engineering has historically been more introverted. There still are opportunities like that. But you must know that the industry is changing. Many projects have adopted Agile Methodology for software development, which requires daily meetings (Scrum Calls) with business analysts, testers, managers, and stakeholders. This puts developers in a high frequency of communication paths. So the introverted isolation for Agile development is no longer in existence. Many projects are adopting Agile methodology or a similar approach where developers are not in isolation. It really is best to learn good communication skills regardless of whether if you are in mechanical engineering or software engineering. This relates to the concept of emotional IQ - the ability to get along with others. Take a course in communication and talk to an experienced engineering advisor. Most people with long-term work experience would agree that communication is very important for engineers.