Arceus is infact the pokemon equivalent of 'god'. Until recently, it was believed that the presence of Arceus was conjecture and entirely made up as the only evidence of their existence was ancient texts and their creation myths.
Arceus and the Trinity
It is commonly held belief and is actually true that Arceus created The Trinity in order to look over Poqmori and its inhabitants. In many respects Arceus pawned off the responsibility for the entirety of the Poqmorian solar system onto these three. Reasons for this are unknown but it is speculated that they wanted to further work on refining other stuff within the universe.
The relationship between The Trinity and Arceus most often seen as one similar to a deity and their angels or messangers. In this particular case The Trinity is more like Arch Angels than anything else. This would actually be correct if anyone could prove it. Other popular viewpoints Is that Arceus is infact the devil and that the Trinity cast them out of Poqmori in ancient times and that their renewed apperance is heralding the doom of this world. Another view is that Arceus is infact a legendary bounty hunter, tracking down evil doers and bringing them before the Trinity to be judged. In this case it is believed that Arceus is the only being that can stand before the Trinity without being judged themself.
Arceus and Legendaries
Regardless of belief structure ALL faiths hold that Arceus is infact more powerful than any of the lesser legendaries. Even more powerful than most of the greater legendaries (Ho-oh, Lugia, ect...). Regardless some believe that Darkrai and the recently ascended Nuhvok to be in league with eachother as some sort of Unholy Trinity.
Truth be told, Arceus is the only entity in this universe that can create or destroy even a single unit of energy, even that contained within the legndary pokemon. Doing this would ofcourse have disasterous concequences for the universe and result in its collapse, causing Arceus to have to rebuild everything from scratch. So being that their modus operandi is creation, they demolish the old energy and create something new with it. This is effectively spiritual reincarnation for legendaries.
Arceus and Avatars
Arceus does infact have an avatar. They are unique in the fact that their avatar is still subjected to aging and will infact die of old age. The uniqueness comes from the fact that Arceus takes their soul complete with memories and reincarnates it into a new body. The soul ends up with a new personality regardless. The avatar of Arceus has absolutely no special powers whatsoever, if you met them in the street you would not be able to go 'hey look theres an avatar' even other avatars which sometimes are able to sense each other draw a complete blank towards Arceus' avatar. They are 100% completely normal aside from the forceful reincarnation and remembering past lives bit.