It began in Florence in the 1590s, when Renaissance intellectuals invented recitative, a play sung in monody. The first opera house opened in Venice in 1637. by 1690 a format had crystallised. Each scene was divided into two parts: the first, in recitative, advanced the plot; in the second, a dramatic climax produces sung melodies (arias).
Artificiality is the breath of life to opera. The more extravagant, the better. Each new addition, whether Gluck's use of ensemble and chorus or Mozart's grand finales, has inflated the experience and heightened the emotional temperature. No wonder opera design is so startling.