The upnishads are late Vedic and post Vedic Sanskrit texts that document the transition from the archaic ritualism of vedas into new religious ideas and institutions and the emergence of the central religious concepts of Hinduism

There are around 200 upnishad and on basis of vedas it is 108 in the upnishads mention of god bram and soul nature and relation is described in very knowledgeable way in upnishads knowledge is about Vedic yagya is described very deeply

In upnishads god is referred as Brahmin the upnishads main quality is that they always focus on truths in upnishads it is mentioned that soul is forever the four principles of upnishads are dharm moksh Karma and the world upnishads are so deep that they are a river of knowledge