How did Giovanni Pierluigi da Palestrina treat plainsong in his hymn settings?

In his hymn settings, Giovanni Pierluigi da Palestrina typically paraphrased the traditional plainsong melody, usually placing it in the highest voice.1

This approach contrasts with his treatment of other sacred works, such as his masses, where he frequently employed the cantus firmus technique—using a preexistent melody as a tenor voice—or the parody technique, which utilizes existing musical material as a starting point for a new composition. While Palestrina demonstrated a remarkable ability to ornament and decorate solemn plainchant in some of his masses, making it an integral part of the musical texture, his specific practice for hymns remained focused on this paraphrasing method.1