Learn Finnish: Hän pyytää lupaa pomolta. - He asks the boss for permission.

The ending -lta is the ablative case, which translates to "from" or "from someone". In Finnish, when you ask a person for something, you literally ask it "from" them. So "from the boss" becomes pomolta.

How would I ask "Who does he ask for permission?" using this structure?

You would use the ablative question word keneltä (from whom). The question would be Keneltä hän pyytää lupaa? Notice how the -ltä ending on the question word perfectly matches the -lta ending on the answer, pomolta.

Why is the word for permission lupaa instead of just lupa?

Lupaa is in the partitive case. The verb pyytää (to ask for / to request) usually requires the thing you are asking for to be in the partitive case, as you are asking for "some" permission rather than a specific physical object.

Does the verb pyytää change its stem when I talk about myself?

Yes! It is a Type 1 verb with a strong "t" sound in its basic form. For the "he/she" form (hän pyytää), the "t" stays strong. However, if you want to say "I ask", the "t" weakens to a "d" due to consonant gradation: minä pyydän.