Why are all Moscow church buildings designed with onion tops?

It's an architectural style - Baroque. It is a style developed during the Italian Renaissance in the eighteenth century. Many churches in Russia are baroque. The onion domes, which appear alone, represent Jesus; those that appear in groups of three represent the Holy Trinity; and those that appear in groups of five represent Jesus Christ and the Four Gospels.

The onion dome (Onion Domes ) is, as its name describes, a dome with the appearance of an onion. The diameter of this type of dome is usually larger than the drum-shaped seat that supports the dome. And their height usually exceeds their width. It is the dominant style of Russian church domes.

Predominantly found in Russian Orthodox churches, Bavaria, and Germany, but can also be seen in the Mughals, India, Austria, Eastern Europe, the Middle East, and Central Asia.

Stone buildings began to appear after the acceptance of Christianity at the end of the 10th century under Byzantine influence, but were mostly used for public ****ing buildings. However, the Russians have retained their traditional wooden construction techniques, which are mixed with stone construction. Wooden buildings are made of very simple materials. Wooden buildings can be built with simple techniques (such as mortise and tenon joints, bolting, etc.) and little labor, and can be used to create beautiful and functional dwellings, with the flexibility to change the complexity of shapes. The small wooden church in the picture is located in Suzdari, so you can imagine some of the characteristics of wooden architecture.