Is it generally true that igneous rocks are formed by the crystallization of molten magma?

Yes, it is generally true that igneous rocks are formed by the crystallization of molten magma. When magma cools and solidifies, it can form intrusive igneous rocks, such as granite, if it cools slowly beneath the Earth's surface. Conversely, if magma erupts as lava and cools quickly on the surface, it forms extrusive igneous rocks, like basalt. This process of crystallization is fundamental to the formation of all igneous rocks.