"그녀는 산에 올라가요."
Translation:She goes up to the mountain.
Because it is a compund word from 오르다 (climb) + 가다 (go) =올라가다. (Learn 르 irregular firstly to understand this)
So it has new meaning: goes up.
"올라가다" isnt confined to the meaning of vertical ascension, in this case it means to go up as in north. if she were doing the action of going up on the mountain, the particle would be 에서.
though there is that obscure verb 등산하다 for mountain climbing, specifically.