May be a silly question but what is the exhibition and why do you enter? Also what decides which pet wins?

The exhibition is a weekly contest (that was paused for a while, but was started again a few days ago) in which players can enter their pets in a contest for certain characteristics, which are different each week. For example, this week's theme is impeccable winners, which counts the number of Arena wins and losses. You can apply here (it costs a small amount of silver to apply), and applicants will be ranked. At the end of the week, players win prizes according to their ranking, and a trophy achievement will be awarded to ranking 1-5.

If it is still unclear, don't be afraid to ask questions!