Finale Confusion

Wait, hold up, I understood the finale right up until now. So the ending scene of them in the church reveals there in purgatory, and are about to move onto the the afterlife. Explain to me then how Desmond was able to bring his consciousness into the sideways story??? When looking at it as it being an alternate timeline it makes sense. But I just realized, that the sideways stuff is essentially purgatory, so you’re telling me he put his consciousness into purgatory??? Somewhere where he is dead??? I’m sorry I’m so confused now please help