The Silver Flame is a gestalt celestial force that was forged in the Age of Demons, when the couatl sacrificed themselves to bind the overlords. Its power has been wielded by many champions across the ages, across the world. The Silver Flame is not a god. It was created for an explicit purpose: to be the cage that keeps the apocalypse at bay. It cannot influence the world on its own but can empower mortals to fight for it. Ultimately, it is a resource to be used to fight against evil. The Silver Flame protects the entire world from the overlords. It doesn't elevate any culture or ancestry, and by extension, those who wield its power should protect everyone from evil.
It could be that the Silver Flame is exactly what it seems to be, a gestalt celestial entity formed from the collective immortal essence of most of Eberron’s couatl. It is a force that permeates the world, like gravity, and while there are locations where expressions of it are visible, it’s not a thing you can really see or visit.
The exact nature of the Silver Flame isn’t known, but there are likely multiple theories debated by scholars. One such theory is that the Flame is a vast, continuous metaconcert maintained by the couatl. The argument for this theory is that the couatl were psionic, and often blurred the line between the psionic and the divine. The existence of the shared dream of the Masvirik'uala seems to support this theory: as far as anyone knows, they don’t go to Dal Quor when they dream, so are likely “dreaming” within the Flame.
It is also possible that the Silver Flame blankets the Material Plane in a manner like the Border Ethereal and could be considered its own (artificial) plane of existence. This theory also allows for the Masvirik’uala to dream within the Flame. Planar-powered magical phenomena are not unheard of on Eberron. Irian and Mabar are examples, with energy from both planes powering Aereni deathless and undead respectively. This theory also allows for the possibility of travelling or seeing into the Flame itself.