To explain this you need to know that categorically, there isn't one Vishnu. The original progenitor has 3 Purusha Vishnu avataras at each level. Maha Vishnu (Karanodakshayi Vishnu) at the multiversal level, Garbhodakshayi Vishnu at the universal level and Ksheerodakshayi Vishnu at the microcosmic level. The 3 Vishnus combined is no less different than any one Vishnu or even the progenitor Itself.
(Chaitanya Charitāmṛit, Ādi Leela 4.8)[v5]
“Shree Krishna does not directly involve himself in the works of
creating, maintaining, and dissolving the material universes.” Shree
Krishna’s primary activity is to engage in eternal loving pastimes
with the liberated souls in Golok, his divine abode. For the purpose
of material creation, he expands himself as Karanodakshayi Vishnu, who
is also called Maha Vishnu.
Maha Vishnu is thus the form of the Lord that presides over the
material realm, consisting of infinite material universes. Maha Vishnu
is also known as Pratham Puruṣh (first expansion of God in the
material realm). He resides in the divine water of the kāraṇ (causal)
ocean and manifests innumerable material universes from the pores of
his body. He then expands himself to reside at the bottom of each
universe as Garbhodakshayi Vishnu, who is called Dwitīya Puruṣh
(second expansion of God in the material realm).
From Garbhodakshayi Vishnu, Brahma is born. He guides the process of
creation—creating the various gross and subtle elements of the
universe, the laws of nature, the galaxies and planetary systems, the
forms of life residing in them, etc. Hence, Brahma is often referred
to as the creator of the universe. However, he is actually the
Garbhodakashayi Vishnu further expands himself as Kshirodakshayi
Vishnu, and resides at the top of each universe, in a place called
Kṣhīra Sāgar. Kshirodakshayi Vishnu is also known as Tṛitīya Puruṣh
(third expansion of God in the material realm). He resides at the top
of the universe, but he also resides as the Supreme soul, in the heart
of all living beings in the universe, noting their karmas, keeping an
account, and giving the results at the appropriate time. He is thus
known as the maintainer of the universe.
All the three forms of Lord Vishnu mentioned here are non-different from Shree Krishna. Thus, in this verse, Shree Krishna states that all spiritual and material creation emanate from him. Shree Krishna is also called the Avatārī (the source of all the Avatārs). The Śhrīmad Bhāgvatam states: ete chāṁśha kalāḥ puṁsaḥ kṛiṣhṇas tu bhagavān svayam (1.3.28)[v6] “All the forms of God are the expansions, or the expansions of the expansions of Shree Krishna, who is the primordial form of God.” And so, the secondary creator Brahma prays to Shree Krishna:
jīvanti lomavilajā jagadaṇḍanāthāḥ
viṣhṇurmahān saihayasya kalāviśheṣho
govindamādi puruṣaṁ tamahaṁ bhajāmi (Brahma Samhitā 5.48)
“Infinite universes—each having Shankar, Brahma, and Vishnu—manifest
from the pores of Maha Vishnu’s body when he breathes in, and again
dissolve into him when he breathes out. I worship Shree Krishna of
whom Maha Vishnu is an expansion.”
One Vishnu looks after maintenance. But when we get a closer understanding, we come to know about three purusha avatars. Normally whenever the word Vishnu is refer to that does not necessarily refer to Vishnu of spiritual world because we do not have direct contact with Vishnu of spiritual world. The Vishnu whom Brahmaji and devatas go to appeal for protection, when the earth is disturbed that is Ksheerodakshayi Vishnu. There is Maha Vishnu, who is just like the super soul of whole cosmos. The super soul of each universe is Garbhodakshayi Vishnu. The super soul of each living being is Ksheerodakshayi Vishnu.
Garbhodakshayi Vishnu is the source of the lotus on which Brahmaji appears. Now that same Garbhodakshayi Vishnu expands and he has his loka. Brihad Bhagvatam talks about it and Brahmaji worships that particular devata. He is the aradhaya dev. Then beyond this fourteen planetary system, above Brahma loka there is in the vicinity but further away there is ksheerasagar and in beyond that is the abode of Ksheerodakshayi Vishnu.
Normally whenever the devatas are going and praying to Vishnu for protection it is that Vishnu that is being referred. That is Ksheerodakshayi Vishnu. The other forms of Vishnu are not directly accessible to us because Maha Vishnu is completely beyond the universe and even Garbhodakshayi Vishnu at the bottom and generally go at the most to the lower planets or to hell but not below then that. Therefore, in normal conversation Vishnu referred to is Ksheerodakshayi Vishnu and he functions as the maintainer. The same Ksheerodakshayi Vishnu is present in the heart also and he keeps the functioning of the heart and the body going.