Paella is originally from Valencia, but prepared all over Spain, and that leads to some local variations and creative results. International exposure to tourists and foreign cooks has created an even bigger variety of paellas.
But since some years ago, there has been a growing group of Valencians that have been very outspoken (with varying levels of seriousness) about what goes into a paella, and what makes it just an "arroz con cosas" (rice plus things).
They even successfully lobbied to have a correct paella icon in unicode!
https://emojipedia.org/shallow-pan-of-food/ But, to great jest, a lot of implementations got it wrong (only apple, twitter, and emojipedia get it right in the previously link).
Supposedly, paella just adds chicken, rabbit, green beans and habones (big white beans) to a saffroned rice.
In addition to ingredients, the real distinguishing factor that makes paella paella and not "arròs amb coses" is that the entire dish—including the broth—is prepared from scratch within the paella (the pan, outside of Valencia they're called paelleras). The best online recipe I've found for the whole process is: http://www.lapaella.net/receta-paella-valenciana/
The response in twitter was very funny