They started by putting the texts in context - the oral versions date from about 700BC and are the last part of the Vedas, which are the rituals performed by Hindu priests. The Vedas are part of the ancient concept of religion as control of the world - these rituals are spoken in the right way at the right time, with the right ceremonies, and then the gods and world will become ordered in the way you desire. The Upanishads were developed during a time when the tribal societies of the Indian subcontinent were starting to coalesce into kingdoms, with larger urban centres, and are concerned with the meanings and knowledge behind the rituals. They're presented mostly as a series of dialogues between pupil and teacher (with the roles of teacher & pupil being taken by various different people - sometimes father & son, sometimes husband & wife, sometimes King and sage (in either role)). I'm not quite clear on why they started to be written down, perhaps it was just a more general transition from oral to written culture? But even after they were first written down they were still for the priestly class, not for general consumption. Over time commentaries on them were written by religious leaders, and closer to modern times they were translated first into Persian and then into Western languages & became more widely known.
There was an interesting division between the experts. Brodbeck seemed to concentrate on how the texts were about knowledge and about how to transmit and to learn that knowledge. And the other two were more interested in what the texts had to say about the Hindu beliefs about the nature of the universe. Interestingly they were saying that the Hindus were not interested so much in "who created the world" like many other religions, but more in what came before there was a world and before there was a creator - this is the concept of Brahman (I think) which is the universal cosmic power & is described using many different analogies in the Upanishads. They also discussed the desire for immortality reflected in what the Upanishads said, and how this is different from the Western concepts of immortality. In our culture immortality is about the continuation of the personality - either living forever or dying and going to an eternal afterlife as yourself. But in the Hindu religion it can be about the immortality of one's lineage - one's children are one's immortality, they carry on the line. Or it can be about the immortality of the Atman (which again is described with many analogies in the Upanishads but roughly translates as the self). And this isn't your personality, if the Atman is reincarnated the new life isn't related to the old one & doesn't remember it or anything, even tho it's the same immortal Atman. And a goal is to die finally and become part of the Brahman, in an immortal existence that has no more personality or suffering like there is in the world.