The word "穹窿" is formed by combining two characters, each contributing to the meaning of an arched structure. "穹" incorporates the radical "穴" (cave) and the component "弓" (bow), suggesting an arched shape, while "窿" also uses the radical "穴" and the component "隆" primarily for phonetic value. Together, they semantically emphasize a dome or vault, such as the sky or a ceiling, through their shared reference to a hollow, arched form.