They never addressed that in too much detail, but with the focus that there's in regards to avatar, the physical Pokémon Arceus is a manifestation of the original heart that manifests itself as the everything that there's in the world, maybe the physiacl Arceus is the "agent of chalange" that either chalanges humanity or helps them when it's needed. But at that point, I don't see the two as separated.
Seem I didn't ask the question properly. Which is it?
1. The Heart came into existence, and from its awareness directly came Dialga, Palkia, and others
2. The Heart came into existence, and from its awareness Arceus was born, and from Arceus directly came Dialga, Palkia and the others
It seems to be option 2, as it's mentioned several times that Arceus(Pokémon) is the actual creator of the world, the one who came out of the egg specifically, an event that was repeated in Platinum. But at the end of the day, everything that exists are inner workings of the Heart that causes the world to expand as it enriches or smthn like that.
I mentioned this on your twitter but Pokémon the cosmology closely resembles Hinduism, where Brahman (Primordial Consciousness, which is literally what the Heart is) is too perfect to actually need to directly create the Universe , and so he manifests Ishvara, his manifested form, to actually create and maintain the universe, making 2 aspects.
Nirgua Brahman= The Heart that cannot really be described by anything (Apophatic). It's never directly mentioned in Pokémon but if Arceus(Avatar) is the direct Origin of all concepts including Nonexistence, and he placed himself in contrast with the absolute nothingness(Chaos) state that the multiverse was, this puts the Heart in a grey area where it doesn't fit any categories of things that exist and do not exist . This is especially true because there are even abstracts (Voice of Life) and Nonexistent beings (Shadows of Nothingness) within the multiverse that Arceus maintains and is the Origin of.
Saguna= The manifestation of the Heart that is the actual creator of the universe.