Mandukhi Upanishad says you and the world that you experience are one reality. It's very easy to understand this if you take the example of our dreams, your individual dream. You yourself dream up a universe where the totality is nothing other than your mind in that universe. Then we find ourselves as one individual in a vast world of many, many individuals.
Swami Sarvapriyananda teaches one of the shortest and most profound upanishads accompanied by Gaudapada's Karika. This lecture covers Karika 1-5.