"уходить" is to leave. "выходить" is to exit "выходить" also means to leave temporarily, notably for a short period of time. Example: "уходить из дома" - to leave from home "выходить из дома" - to exit the house "выходить, чтобы вынести мусор" - to leave the house in order to take out the garbage

If you вышел, it means you were inside and now you are outside (and maybe you'll be back soon). If you ушел, it means you were here (no matter inside or outside) and now you are not (and you won't be back soon).

thank you very much for the further information. it is really clear

Вы as prefix expresses, as Dmitri has already said, the temporary action of going OUT, it refers to the fact you were just "inside" and now you are outside a space; У has another meaning, "away". When you say " уходи(те)!" to someone , you want him/her to go away, not just outside. If u want to know if someone is still in a place or he/she has already left, you use уходить Где Иван? Он уже ушел? In the same way, we use y- to express "leaving a place" (departure) Когда ты уезжаешь из России/ в Италию/ на Сицилию?