Brahma and Vishnu once entered each other’s bodies. When Vishnu had seen everything in Brahma, he came out and Brahma entered Vishnu’s body.
Vishnu closed the exits of his body, Brahma could not find a way out, he made himself tiny. A lotus sprouted from Vishnu’s navel, Brahma crawled up the stem and sat on the flower. So the lotus (padma) is the birthplace (yoni) of Brahma and he is Padmayoni.
Vishnu is Padmanabha, the one from whose navel a lotus grows.