What makes creme brulee set?
I would prefer to give an answer that doesn't involve spending hundreds of dollars, but Modernist Cuisine has a great table about the consistency of custards comparing cooking temperature to egg concentration. If you can find a copy at your local library the kitchen guide has a table on page 233, otherwise check out volume 4 page 84.

Quick synopsis: What sets the creme brulee is the egg proteins coagulating. If you were to take the weight of the liquid in the brulee and add 30% of that weight as eggs and cook it to 181 degrees F you would have a creme brulee texture. Overcook to 190 F and you have flan, undercook to 176 F and you have creme anglaise. The two main factors are egg concentration and cooking temperature.