Purusha, dwelling in the body, witnesses, permits, supports and experiences everything. The overlord of all, He is also said to be Paramātmā, the supreme Self.
Krishna has already stated that Kshetrajna is distinct from kshetra that consists of the whole creation, including our body and all changeful experiences in it. Here he says that the Purusha dwelling in this body is truly the transcendental, supreme Self, called Paramātmā. He is the unchanging witness as well as the sovereign Lord of all the activities and experiences.
Prakrti remaining the object, outer and inner, Purusha shines as its distinct Subject. Despite this distinction, Purusha permeates throughout Prakrti, causing and presiding over all that transpires in it. That such an indwelling presence is there, involved in but distinct from all changes, will be clear, if we enquire into ourselves.
The ‘I’ shines, involved with everything the body does, but ever distinct. The ‘I’ indicates this Purusha inhering in Prakrti, but distinct from it, as the singular Subject, the supreme Self.