Upanishad literally means “to sit at the feet of the Master and listen to his words or teachings.” The Upanishads constitute the philosophical and spiritual essence of the Vedas and transmit the highest knowledge about God, the soul, and the world, as well as the importance of ceremonies and sacrifices. They are mostly written in the form of questions and answers between a disciple and his teacher.
In order to study the Vedas in detail, six texts are used, known as the Vedangas. The six Vedangas are: Shiksha, Kalpa, Vyakaran, Nirukta, Jyotisha, and Chhanda. The Samhitas and the Brahmanas form the Karma-Kanda segment of the Vedas and are concerned with rituals and ceremonies to achieve earthly desires. The Aranyakas and the Upanishads form the Jnana-Kanda segment of the Vedas. They focus explicitly on philosophy and spiritualism.
Vishishta Advaita
According to the followers of this doctrine, there is only one reality, Brahman, qualified by two "modes" (aspects or attributes): sentient beings and insentient beings. The creator of this doctrine was the religious Yamuna Acharya (10th-11th century) and its propagator was Ramanuja Acharya (1077-1157), who established the identification of a personal God with the impersonal divinity Brahman (who appears as the subject of the Upanishad scriptures).
For him, the soul and matter depend on God for their existence. Exclusive devotion to God can awaken his grace, helping the devotee to gain liberation from the cycle of reincarnation.
After Ramanuja, a schism developed over the concept of grace, giving rise to different schools. The dissemination of the Vishishta Advaita doctrine was important in the devout revival of Vishnuism (worship of the god Vishnu) - which in southern India continues to have a significant influence - and especially of Krishnaism (also called Gaudíya Vaishnavism: 'Vishnuism of Bengal', the worship of Krishna).
To know more, visit https://members.vedalearn.net/posts/exploring-the-vedas-upanishads-53566335