Sri Aurobindo and his disciples uncovered connections between the Vedas and the later scriptures such as Upanishads, Puranas and the Tantra by tracing the evolution of concepts, use of common verses and the underlying symbolism between these scriptures. This is a synopsis of their discoveries collated from a variety of sources.
The Upanishads, besides delineating various spiritual experiences, also give a few hints on sadhana, i.e., paths of spiritual realization. These methods of sadhana are called vidyas. This post outlines this in brief.
This post describes the three knots/granthis of mental, vital and physical ignorance that tie our consciousness to the physical world and bind our soul to the superficial personality. When these knots are broken, our consciousness widens and opens to the cosmic mind, vital and physical.
The Vedas and the Upanishads speak of a golden lid (Hiranmaya Patra in Sanskrit) which divides the lower rational mind from the higher planes of the Mind above us. This post explains the significance of that Golden Lid in the words of Sri Aurobindo & The Mother.
The ancient Hindu scriptures speak of the Universe as an inverted Banyan/Ashwattha tree with its roots in the Higher Worlds of Sat-Chit-Ananda (Existence-Consciousness-Bliss) and its branches in the many lower worlds that have been created. This is a comprehensive list of references to this tree in the ancient scriptures and the works of Sri Aurobindo and The Mother. The world tree motif is present in many other religions and mythologies (See World-Tree)