Short answers, pulled from the story.
According to historian Wolfgang Born, the onion dome originated in Syria, where Umayyad Caliphate-era mosaics show early stages of bulbous dome development. An early prototype also appeared at Chehel Dokhter, a mid-11th-century Seljuk structure in the Damghan region of Iran.
Several explanations have been proposed. Scholar Sergey Zagraevsky argued that the elongated dome shape allowed builders to place a heavy stone inside as ballast for Russia's large, elaborate crosses, which were too heavy for flat Byzantine dome structures. A separate tradition holds that the shape symbolizes a candle flame, expressing prayer as a soul burning toward heaven.
Buildings featuring onion domes appeared in Russia as early as the 13th century. Historian Boris Rybakov, writing in 1946, found that most Russian chronicles from the 13th century onward depict onion rather than helmet-shaped domes. Scholar Sergey Zagraevsky identified onion dome images on icons dating to the 12th century.
Saint Basil's Cathedral was built by Ivan the Terrible to commemorate his conquest of the Khanate of Kazan in 1552. Some scholars believe Ivan adopted the onion dome form from that Muslim territory. The cathedral's domes have not been altered since the reign of Ivan's son Fyodor I, confirming the dome's presence in 16th-century Russia.
In 1917, religious philosopher Prince Evgenii Troubetzkoy argued that Russian church drums were deliberately shaped to resemble candles, expressing a religious and aesthetic attitude he linked to the folk phrase "glowing with fervour." He described the Ivan the Great Bell Tower as a gigantic candle burning above Moscow and the Kremlin's clustered domes as enormous chandeliers.
Onion domes appear on Catholic churches across southern Germany, Switzerland, Austria, the Czech Republic, and northeastern Italy. The first example in that region was built in 1576 in Augsburg by architect Johannes Holl. The form is also found in Mughal, Sikh, and Rajput architecture in South Asia, in mosques across Indonesia, and on the Corn Palace basketball arena in Mitchell, South Dakota.