Bhashya
तत्कथमवेदित्याह—अहं द्रष्टुर्दृष्टा आत्मा ब्रह्मास्मि भ-वामीति ।ब्रह्मेति— यत्साक्षादपरोक्षात्सर्वान्तर आत्माअशनायाद्यतीतो नेति नेत्यस्थूलमण्वित्येवमादिलक्षणम् ,तदेवाहमस्मि, नान्यः संसारी, यथा भवानाहेति ।तस्मात्एवं विज्ञानात् तद्ब्रह्म सर्वमभवत्-अब्रह्माध्यारोपापग-मात् तत्कार्यस्यासर्वत्वस्य निवृत्त्या सर्वमभवत् ।तस्माद्यु-क्तमेव मनुष्या मन्यन्ते-यद्ब्रह्मविद्यया सर्वं भविष्याम इति ।यत्पृष्टम्-किमु तद्ब्रह्मावेद्यस्मात्तत्सर्वमभवदिति, तन्निर्णीतम्--ब्रह्म वा इदमग्र आसीत्तदात्मानमेवावेदहं ब्रह्मास्मीतितस्मात्तत्सर्वमभवदिति ॥
In plain words
Explaining how Brahman realized the Self, the text says: 'I am the ultimate witness, the Self, Brahman.' That which is immediate, beyond the senses, residing within all, transcending hunger and thirst, defined solely through the great negations ('not this, not this')—that very reality am I, not a limited transmigrating soul. Therefore, through this direct realization, Brahman became everything. By removing the false superimposition of non-Brahman and destroying the finitude born of ignorance, it attained universality. Thus, human aspiration is entirely justified when people declare, 'Through the knowledge of Brahman, we shall become all.' The initial question—'What was that Brahman which it knew, whereby it became all?'—has now been fully resolved: 'In the beginning, this universe was Brahman. It knew only itself, saying, "I am Brahman." Therefore, it became all.'
Close to the Sanskrit
How it knew, he says: 'I am the seer of the seer, the Self, Brahman, I am'—thus. Brahman, which is directly beyond senses, innermost of all, the Self, transcending hunger etc., having the characteristic of 'not this, not this', neither gross nor subtle etc., that indeed am I, not another transmigrating soul, as you speak. Therefore from such knowledge that Brahman became all—due to the removal of the superimposition of non-Brahman, by the cessation of its product of finitude, it became all. Therefore it is entirely proper that men think: 'By the knowledge of Brahman we shall become all.' What was asked—'What indeed was that Brahman which it knew, from which that became all?'—that has been ascertained: 'Brahman verily was this in the beginning, then it knew the self alone: I am Brahman, therefore that became all'—thus.