What is the best methodology to select a major? (Diploma and academic courses)
You first must determine your values, as ultimately any answer you'd find convincing must satisfy your values. Then, the problem becomes more a question of how to best satisfy these values.

Decision making skills can be useful in choosing a major, and will be useful for almost any choice you make. I'd recommend taking a decision analysis class, as I did during my PhD. The principles involved with making good decisions will be valuable in almost any endeavor.

If you want to improve the world in a very utilitarian way, the website 80,000 Hours should prove useful. The organization is affiliated with the University of Oxford and focuses on helping young people make valuable and satisfying career choices. Of course, education is a major part of this. They have a career guide that I've read a large fraction of, along with a decision tool.