Happened in a real game: my opponent casts a show and tell, in response I cast seed time, he gives me the okay that it resolves. Then we resolve show and tell, he puts omniscience into play, and casts emrakul the aeons torn. Who takes the extra turn first? Bonus: if he had emrakul the promised end which of my turns would he take over?
The most recently created extra turn is taken first, so your opponent will take their Emrakul turn before you take your Seedtime turn. If it’s easier for you to visualize, you can think of extra turns having their own stack where they pile up waiting to be taken.
In the case of player-controlling effects, they apply to the next turn the chosen player would take, so your opponent would control your Seedtime turn, then you’d take the extra turn from Emrakul 2.0 and then your normal turn.