What I think is that the cosmic Vishnu, who sees through the work of the multiverse is responsible for this cosmic travel. So, he basically must have sent a interstellar spaceship and he was the pilot. The people compared this to a boat and a fish navigating through the cosmic sea and that's what they told their children of course because it was difficult to explain all these things when you don't have any technology of that sort.

Lord Brahma had created a tunnel from one universe to another. There is a theory that considers universes as parallel sheets like the sheets of a newspaper. What I think is that they travelled among these sheets to a universe where the earth was already ready and habitable.

Sheshnag actually guided them through the external disturbances, hence protecting them. That's why it is mentioned that a huge snake had guided them in this cosmic travel.