These Viṣṇu, Kāraṇodakaśāyī, Garbhodakaśāyī, and Kṣīrodakaśāyī, these are little technical. Try to understand. Now this universe, which you find just like a big ball, there are innumerable universes like this, and they are floating in water. That water is called Causal Ocean. So that Causal Ocean there is, I mean to say, completely in the whole ocean, big gigantic body is lying down there. That is known as Kāraṇodakaśāyī Viṣṇu. The Viṣṇu in that gigantic form is sleeping within the water of that Causal Ocean, and by His inhaling and exhaling, breathing, there are bubbles. And those bubbles are manifested as universes. This is stated in Brahma-saṁhitā: yasyaika-niśvasita-kālam athāvalambya jīvanti loma-vilajā jagad-aṇḍa-nāthāḥ [Bs. 5.48]. That breathing, exhaling and inhaling... Just like we also do that. So we have inherited that exhaling-inhaling, from that Viṣṇu.
So how these brahmāṇḍas or universes, each of them has a predominate deity? He is called Brahmā. Brahmāṇḍa, the master of this brahmāṇḍa is called Brahmā. The duration of this Brahmā, duration of life, is only the inhaling and exhaling period of Mahā-Viṣṇu. When He's exhaling, all the brahmāṇḍas are coming in existence. And when He's inhaling, they're all vanquished. This process is going on. And the Supreme Person who is doing this, I mean to say, creation and dissolution, He is called Mahā-Viṣṇu. Viṣṇur mahān sa iha yasya kalā-viśeṣo govindam ādi-puruṣaṁ tam ahaṁ bhajāmi **. So the reference is in the Brahma-saṁhitā that that Mahā-Viṣṇu is also part and parcel of Govinda, Kṛṣṇa. Just try to understand what is Kṛṣṇa.
Prabhupada's lecture - Śrīmad-Bhāgavatam 1.3.1-3 San Francisco, March 28, 1968