Thanks.

This explains how they are different, but I am more interested in why they are different.

GMs make better moves than amateurs, they make them more quickly, they are able to memorize positions more easily, they have a more accurate intuitive understanding of positions, but why?

Is it the number of hours spent playing chess, the number of hours spent analyzing positions, the number of hours spent doing tactics puzzles, or something more general or more specific?

Is there any way to predict a GM, or better yet make a GM, with something measurable?

What is the difference between a 1400-rated player and a 2600-rated player in terms of their chess education, their experiences, their past environment, their general mental or psychological qualities.

I am asking not so much about what abilities GMs have that amateurs don't, but about why they have these abilities. Is it how many games they have played? Is it their memory? Is it their psychology? Does 10,000 chess hours = GM?

I assume there is more than one requisite, and more than one way of achieving the necessary knowledge (positional knowledge can be learned from a coach, or can be learned from a book, or can be learned through self-study while reviewing one's own and others' games) but I would like to know what all chess masters have in common, what if removed would cause a master to start losing all of their games to other masters.

I asked an NM what he thought was necessary to become good at chess, and his answer was "a great dislike for losing, a mentality of wanting to continually improve, where one does not become discouraged, but rather encouraged to learn from their mistakes".

I asked an FM whether he had read My System or studied any other positional books, and he said that his chess education consisted solely of doing tactics puzzles and playing many correspondence games with the help of an opening encyclopedia.

An IM told me that he never had a chess coach until after he became a master, and mostly became fluent in chess by moving the pieces around on the board and looking for patterns by himself.

I assume there are many routes to becoming a GM, but what do they all have in common? And is there a GM who started playing chess as an adult and then became a GM? How did they do it?

On a side note, someone mentioned Silman, and others talked about differences between IMs and GMs, but I understand that distinguishing between multiple titles (GM, IM, etc) is somewhat arbitrary and that once someone becomes a chess master (by mastering all of the elements of chess) they could become an IM or GM given enough play, and that the main barrier is not as much related to progressing their ability from IM- to GM-level, but more related to how much time they can spend traveling and attending tournaments etc in order to improve their rating and get GM norms. Is this correct?