Would it be dangerous if you put air up your urethra through a straw and it was way into your penis would it go into your kidneys and if so why would the kidneys be sore after?

Air by itself would not likely cause a problem. The oxygen would be absorbed over time ant the other gas would likely diffuse. The act of introducing this through a foreign object like a straw could easily introduce germs that could infect/damage any location within the urinary tract including the kidneys. Best to avoid actions that could do such damage.

The manipulation you described puts at risk of rupture or urethra and urinary infection. Quit doing crazy stuff like that.

No it will not go up to your kidneys. Kidney area soreness is not related to this activity. This activity can be very harmful and I advise stonngly against it