How does the jet streams develop?

I've tried to understand how the jet streams develop....but all the internet doents and videos i've watched and read just tell me what they do. Can someone please explain to me how exactly the jet streams develop, or where do their speeds come from? I know that they start off as air in between two corresponding cells(hadley, polar, ferrel)....but i really don't understand how the jet streams develop their speed.....please help me! thanks guys