Who decides what “AI” is and how did we come to decide what fits that title?

Language is ever-evolving, but a good starting point would be McCarthy et al., who wrote a proposal back in the 50s. See http://www-formal.stanford.edu/jmc/history/dartmouth/dartmouth.html

Techniques have come into and gone out of fashion, and obviously technology has improved, but the principles have not fundamentally changed.