Bhagavad Gita · Jnana Karma Sannyasa YogaChapter 4, Shloka 6 (4.6)

Three "even though"s. Aja (unborn), avyayātmā (imperishable), bhūtānām īśvaraḥ (master of all) — even though he is these, he takes birth. This is the secret of the avatar: even with birth, the nature of the Lord is not lost.

Shankaracharya — "as if born". Prakṛtiṃ svāṃ mama vaiṣṇavīṃ māyāṃ triguṇātmikām … adhiṣṭhāya vaśīkṛtya — bringing under control my own Vaishnavi maya made of the three gunas, under whose sway the whole world is and deluded by which people do not know their own self, Vasudeva — sambhavāmi dehavān iva bhavāmi jāta iva ātma-māyayā, na paramārthato lokavat — I become as if embodied, as if born, through my own maya — not in reality, as ordinary people are.

Acharya Ramanuja — a real birth, a divine form. Ajatvāvyayatva-sarveśvaratvādi-sarvaṃ pārameśvaraṃ prakāram ajahad eva — without giving up any of my nature as the Supreme Lord — unborn, imperishable, Lord of all; svāṃ prakṛtim = svabhāva — abiding in my own nature, svena eva rūpeṇa svecchayā sambhavāmi — I take birth in my own form, by my own will. What is that form like? Ramanuja explains from many Upanishadic texts — ādityavarṇaṃ tamasaḥ parastāt (sun-coloured, beyond darkness) and others — that is, a divine, non-material form. Ātma-māyayā = by my own knowledge/will. And he quotes the Yajurveda: ajāyamāno bahudhā vijāyate — though not being born, he is born in many ways — that is, he does not take an ordinary birth, but by his own will appears in the forms of gods, humans and others.