How do wifi tournements work in pokemon and please be as specific as possible.?

I am thinking of starting competitive battle with strangers now that the friends i usually battled live further away after graduation. also im mostly agaist hacking but ar pokemon who have been leveled up with an ar code taboo? My jolteyon and wevile were leved up this way because i dont have time to have no life, mindlessly leavling up my pokes when i have friends to hang out with and study....