How does a musical phrase relate to a theme in a larger composition?

In musical composition, a phrase is a fundamental structural unit—a complete musical utterance that functions as a building block for larger musical ideas. Because musical forms are hierarchical, phrases are conjoined to create a melody, which in turn serves as a constituent part of a larger work.1

A theme is defined as a melodic entity that functions as an element within this larger whole. Essentially, while a phrase is a short, self-contained musical thought, a theme is a broader melodic subject that is often composed of multiple phrases. These phrases relate to the theme through their arrangement, which may involve repetition, contrast, or variation to establish the theme's identity and provide coherence to the composition.1