Short answers, pulled from the story.
The Sanskrit term Upanishad originally meant connection or equivalence and later came to be understood as sitting near a teacher. It derives from upa meaning by and ni-śad meaning sit down, referring to the student sitting down near the teacher while receiving spiritual knowledge.
The authorship of most Upanishads remains unknown because Sarvapalli Radhakrishnan states almost all early literature of India was anonymous. Exceptions exist such as the Shvetashvatara Upanishad which includes closing credits to sage Shvetashvatara considered the author of that text.
The general area of composition for early Upanishads is northern India bounded on the west by the upper Indus valley and extending eastward to the lower Ganges region. These areas form a frontier zone of Brahmanism together with zones immediately to the south and west including modern locations like Bihar Nepal Uttar Pradesh and Uttarakhand.
The Upanishads form one of three main sources for all schools of Vedanta alongside the Bhagavad Gita and Brahmasutras. Schools named themselves after relations they see between Atman and Brahman such as Advaita Vedanta stating there is no difference between soul and ultimate reality or Dvaita considering all individual souls and matter as eternal mutually separate entities.
First translations appeared during reign of Mughal Emperor Akbar spanning 1556 to 1586 when these were rendered into Persian language. The Latin translation introduced Upanishadic thought to Western scholars initially while the first Sanskrit-to-English translation of Aitareya Upanishad occurred in 1805 through Colebrooke.