First principle of the purusha form's composition
The purusha form is 'sambhutam mahad-adibhih' — made of mahat and the other principles (SB 1.3.1).
Concept
Mahat-tattva
महत्तत्त्व
Mahat-tattva is the first manifest principle of the Sankhya-Vedanta cosmology: the 'great' intellect-stuff from which ego and the entire material evolution proceed. In the processed verses the purusha form is 'composed of mahat and other principles' (SB 1.3.1), and maya fashions the Lord's cosmic form beginning with mahat (1.3.30).
Śrīmad Bhāgavata Mahāpurāṇa 1.3.1 — Sanskrit Wikisource (pinned oldid=369178)
The first transformation from the unmanifest; of the nature of understanding
From the unmanifest, driven by time, the mahat principle arose, of the nature of understanding, revealing the universe seated in the self of the body and driving away darkness: 'tato abhavat mahat-tattvam avyaktAt kAla-choditAt' (SB 3.5.27). Then, becoming the object of the Lord's glance, it makes itself manifest (SB 3.5.28), and from it the aham principle emerges (SB 3.5.29).