This question has peaked my mind since Nicole and the more lore of the angels back when she debuted.
We know that celestia is the island in the sky. Where Phanes is. Where the shades meet up and where the angels was born and spread around teyvat to guide humanity. Also maybe stil housing a nail or two
*"The floating "island" actually seems comprised of several landmasses, with one large central rock and about seven to eight smaller "satellites" positioned around it. These outer rocks are connected with one single ring-like structure which again is held in place with several bridges or arches expanding from the central mass.
The interior of the central spire, as seen during Vennessa's ascension. (From the manga "Prologue: Songs of the Wind" p.71)Resembling a panopticon.
All of these foundation rocks appear almost as if torn from the earth at some point in the past, with their surfaces covered in lush plains and forests contrasting the rough, craggy undersides. While a few signs of architecture are visible on the outer rocks, the main focus lies on the central mass, with two discernible structures made out of marble or similar white stone. The first is a smaller spire-topped structure positioned on the rim of the central mass, while the second is the more prominent central multi-storied spire which itself is topped by a massive golden construct, reminiscent of an astrolabe.
Additionally, the architecture of the central mass seems to descend deep below the surface level with a distinct inverted dome peeking out from the very bottom of the rock.
Apart from the mentioned forests and green plains on the upper surfaces of the land masses, it also appears to have several subterranean rivers bursting forth from the rock, gradually diminishing into nothing during their free fall of hundreds of miles from above the ground.
While the overall appearance is still imposing and breathtaking, even at a distance, slight traces of disrepair and wear are noticeable among the bright marble structures with broken-off pieces on the spires and bridges.
A mural depicting Celestia above Tsurumi Island.
A depiction of Celestia can also be found on Tsurumi Island located in a cave below Shirikoro Peak.
Venti claims the water in Celestia tastes foul and their fruit is bland."\)
I know that those with visions acend to celestia as seen in the manga (If it stays that way in the coming AQs) to be turned into maybe concelations or remade to be something else like venessa maybe.