In other words, Śruti means that which has been heard, in the sense of “echoes in the minds of mature seers.” Śruti is the real source of knowledge, because it does not come from humans (apauruṣeya).

A name for the veda śāstra (the texts of the Veda) that emphasizes its preservation and transmission through careful listening through the teacher-student lineage, karṇa paramparā (karṇa means ear, paramparā lineage). Listening has the purpose of investigating and understanding. Only in full understanding has the śruti been heard in the right way. Only the śruti properly taught by a qualified guru to a qualified aspirant (adhikārī) removes ignorance, after which mokṣa can shine freely.

The whole means of knowledge of words (śabda pramāṇa) of vedānta is derived from this, arising in the course of the ages through the elaboration in smṛti and prakaraṇa grantha.

The characteristic of śruti is that they are considered timeless and not invented by humans (apauruṣeya). A human being is by definition ignorant, because a human being who is knowing is not a human being, but consciousness. You have to hear this from the outside. Later, a text like the Bhagavad Gītā (Vyāsa) was written by a human being, from the memory of the non-dual teaching of the Vedas (the śruti). This is called smṛti.

Together, there are three (traya) springs (prasthāna) that together form the basis of vedānta:

1. The Upaniṣads (śruti prasthānam).

2. The Bhagavad Gītā as an independent text, part of the Mahābhāratam (smṛti prasthānam).

3. The Brahma Sūtrāṇi, an analytical study of Upaniṣad mantras (nyāya prasthānam).

Still later, great minds such as Gauḍapāda, especially Ādi Śaṅkara Bhagavatpāda, Vidyaranya and Sadānanda, but also many others, wrote textbooks, the so-called prakaraṇa grantha, intended to teach the means of knowledge efficiently and effectively.

Sometimes these are more technical in nature (Vedāntasara of Sadānanda), sometimes a combination of poetic and didactic (ātma bodha, Śaṅkara).

So we have śruti, smṛti, nyāya, and prakaraṇa grantha as textbooks. The Upaniṣads are part of the Vedas of divine origin under śruti.

You can also say. In me, brahman, there appears ignorance as well as knowledge. The knowledge comes from īśvara to jīva.
The whole means of knowledge of words (śabda pramāṇa) of vedānta is derived from this, arising in the course of centuries through the elaboration in smṛti and prakaraṇa grantha.

The characteristic of śruti is that they are considered timeless and not man-made (apauruṣeya). Later, a text such as the bhagavad gītā (vyasa) was written by a human being, from the memory of the non-dual teaching of the vedas (the śruti). This is called smṛti. Still later, great minds such as Gauḍapāda, especially Ādi Śaṅkara Bhagavatpāda, Vidyaranya and Sadānanda, and many others wrote textbooks, the so-called prakaraṇa grantha, intended to teach the means of knowledge efficiently and effectively. Sometimes these are more technical in nature (Vedāntasara of Sadānanda), sometimes a combination of poetic and didactic (ātma bodha, Śaṅkara).

So we have śruti, smṛti, and prakaraṇa grantha as textbooks. The upaniṣads are part of the vedas under śruti.