Lord Vishnu the original Shakti...

अहमेव च सा गौरी गौर्यैवाहं महाफ़णिन्।

उमाया यान्ति मद्भक्ता उमाभक्ताश्च मे पदं।।

उमया मे चाभेदेन धृतस्त्रीनरवेषयोः ।

उभयोर्भेद मुक्त्वा तु प्राप्नुयान्नरकार्णवम्।। ... (Vrihad Padma Purana)

ahameva ca sā gaurī gauryaivāhaṃ mahāpha़ṇin|

umāyā yānti madbhaktā umābhaktāśca me padaṃ||

umayā me cābhedena dhṛtastrīnaraveṣayoḥ |

ubhayorbheda muktvā tu prāpnuyānnarakārṇavam||

Lord Vishnu said: Oh, the great serpent (Shesha), I

am Gauri, and Gauri is me. The devotees of Uma are

my own devotees, those who venerate Uma reaches

to me only. We are one but have taken the

appearances of male and female genders only. One

who differentiates between me and Uma will drown

in the ocean of Tartarus.

Well, its because according to both shrutis and smritis, Lord Vishnu is none other than the male form of goddess Uma. That is why the another name of ishwar Shiva is Hari Vallabh, meaning the husband/lover if Lord Hari.

Ishwar Shiva and Bhagawān Vishnu are Purusha and Prakriti in tattva. They gave birth to the Hiranyagarbha (aka Lord Brahma). Where Lord Brahma is called Hirayagarbha which means ‘Golden womb/foetus’. A stalk from Lord Vishnu’s navel supports a lotus on top of which Hirayagarbha remains joined (seated).

What does this imply? A foetus when develops within the mother’s body, it always remains connected to the mother’s umbilical cord and through that cord it get’s its life sustaining materials. Lord Brahma is the golden-foetus who is connected to Lord Vishnu’s navel through an umbilical cord called as lotus-stalk. And Lord Vishnu being the sustainer of the universe (which is same as Hirayagarbha), he protects and sustains that Hirayagarbha on his lotus stalk. Madhu and Kaitabha demons attacked Hirayagarbha; Lord Vishnu killed those demons protecting Lord Brahma. On similar lines, whenever there is chaos within Hirayagarbha, Lord Vishnu takes incarnations and protects the worlds contained within Hiranyagarbha.

Sri Rudram of YajurVeda 5th Anuvaka states.:

नमो गिरिशाय च शिपिविष्टाय च । नमो मीढुष्टमाय ... (Yajurveda 4.5).

namo giriśāya ca śipiviṣṭāya ca | namo mīḍhuṣṭamāya ll

Salutations to Girisha, Shipivishta, Midhustama.

He is the giver of many things and thus the name "Midhustama" is given to ishwar Shiva.

Lord Kartikeya in his Sri Rudram Bhasya (in Shiva Rahasya Itihāsa) says that Bhagawān Shiva is called Midhustama because he gave his seed to form Hiranyagarbha and thus as the originator of Lord Brahma.

रेतसः सेक्त्रे दिव्यहिरण्मयस्य च हरेर्गर्भे विधात्रे विधे:।।

retasaḥ sektre divyahiraṇmayasya ca harergarbhe vidhātre vidhe:||

who gave his Hiranyaretas (divine seed) in the womb of Hari and created Vidhata (Brahma).

Again,

Sri Shiva Purana 3:11:40.:

प्रकृतिस्त्वं पुमान्रुद्रस्त्वयि वीर्य्यं समाहितम् । त्वन्नाभिपङ्कजाज्जातः पञ्च वक्त्रः पितामहः ॥ ४० ॥

prakṛtistvaṃ pumānrudrastvayi vīryyaṃ samāhitam | tvannābhipaṅkajājjātaḥ pañca vaktraḥ pitāmahaḥ || 40 ||

40. You (Vishnu) are Prakṛti and Rudra is Puruṣa. The seed is deposited in you. The five-faced Brahmā was born of your umbilical lotus.

Same thing is represented by Sri Adi Shankaracharya in his Sri Shivananda Lahari as follows.:

"Ardha vapusha bharyaatwam, Gunitwam sakkhitha..." (Shivananda Lahari Para-82).

"Lord Vishnu occupied half your body, Became transformed to be your wife (PARVATI), Became a boar to search for you (LINGA), Became your lady friend to serve the nectar (MOHINI)."

🙏 Namo Narasimha 🙏