"Why French Apples are Bananas in England?"

While doing my parcial project I came across exchange rates. I used to think they were very simple: The currency a country has was more valued in in contrast to others based on their buying power and their economic growth; the bigger coin you had, the better.

However, while doing China's research, I found out that China deliberately davaluated it's Renmimbi Yuan "to have a more competitive exportation power". This didn't made sense to me, but then all those Asian currencys came into the ecuation (Yen, Yuan, etc.) and I realized many asian countries had a very low exchange rate, even though many of them had a very large and stable economy, so why is that?

After some research I only found that currency did reflect the buying power of a country, and the currency devaluated with inflation, however, China's and Japan economies are quite strong and stable, so this doesn't make sense. I think something is missing, but the only thing that came across my mind is that these countries devaluated their currency to hold the peg to foreigner currency without taking out the balance, which means give the natonal currency the space needed by the natural raises and downs a currency naturally has.