Why not 'ön mit csinalsz'?
Because ön takes the same verb endings as ő (he, she, it).
A bit like how you say "Is your Majesty ready?" rather than "Are your Majesty ready?" -- a word that means "you, the person I am speaking to" but that takes verb forms for "he, she, it".
if we say: Mit csinalsz? it also means WHat are you doing?
Yes. Speaking to the informal singular you.
mit is mi(what) in accusative case, right?
Thank you very much!
I still don't really understand what "Ön" means...
You, polite form, singular. Plural form, önök. Both take third person verb endings.