Which section of a sonata-allegro movement used small pieces of the main theme in many different keys?

The section of a sonata-allegro movement that employs small pieces of the main theme in various keys is called the "development." In this part, themes are elaborated, transformed, and explored through modulation, creating a sense of tension and complexity. The development often contrasts with the exposition and leads into the recapitulation, where the main themes return in the original key.