The word Vedanta literally means the end of the Vedas and originally referred to the Upanishads. Vedanta was concerned with the jñānakāṇḍa or Vedic knowledge part called the Upanishads.
These were the last literary products of the Vedic period. These mark the culmination of Vedic thought. The Upanishads, the Bhagavadgita and the Brahma Sutras constitute the basis of Vedanta. All schools of Vedanta propound their philosophy by interpreting these texts, collectively called the Prasthanatrayi, literally, three sources. The Brahma Sutras attempted to synthesize the teachings of the Upanishads.
The diversity in the teaching of the Upanishads necessitated the systematization of these teachings. This was likely done in many ways in ancient India, but the only surviving version of this synthesis is the Brahma Sutras of Badarayana. The Upanishads present a rigorous philosophical inquiry in the form of identifying various doctrines and then presenting arguments for or against them.
They form the basic texts and Vedanta interprets them through rigorous philosophical exegesis. Bhedabheda, as early as the 7th century CE, or even the 4th century CE. Some scholars are inclined to consider it as a “tradition” rather than a school of Vedanta.
The history of Vedanta is divided into two periods: one prior to the composition of the Brahma Sutras and the other encompassing the schools that developed after the Brahma Sutras were written. Badarayana summarized and interpreted teachings of the Upanishads in the Brahma Sutras, also called the Vedanta Sutra. Though attributed to Badarayana, the Brahma Sutras were likely composed by multiple authors over the course of hundreds of years.