None of the above, oddly. For me, the generation of a campaign world for every campaign makes it a homebrew. Players submit elements they want in, which are then converted and included into the world. Those elements could be commercially published campaign settings or adventure modules. They are usually backgrounds, a custom race, subclass, item, spell, or other element the players want in the game. But they could be published works too.
What this process does is allow enormous freedom for the players to submit almost anything they want. But it retains the status of a game by incorporating these into game structures defined by the game rules, generating them into the world, and letting the players game them during play.
It's sort of a kitchen sink, anything goes world, but it's really about putting players in the same situation we are in in our world. We don't know if unicorns exist or not in our reality. They could be and we haven't encountered any yet. Or there might not be. In D&D, the players start in a small area with a background of limited knowledge. Right or wrong that is their starting knowledge of the setting, but as they explore it grows. What is in the world is held in either potential or actual status. Actual being the finite portion of the universe they sense at any given moment. Potential being even beyond their imagining.