When I cast another player's prepped spell, whose spell is it for the card text and where does it go?

Question

Accepted Answer

You are the 'you' on the card: you make every decision and your own prepped spells, charges and allies are the ones the text counts. The player who prepped it is treated as your ally. When it is discarded it goes on top of the discard pile of the player who had it prepped. With Reflective Conduit the returned-to-hand step is done by the player who cast it.

Aeon's End: War Eternal

When I cast another player's prepped spell, whose spell is it for the card text and where does it go?