Can anyone with experience give me some hints? I mean is this algorithm they use successful? And is it true about having big gains?
FALSE. Arbitrage trading is meant for INSTANT profits but I guess you might have interpreted like BIG profits. Arbitrage trading is nothing but catching the gaps of price levels. No big algorithms are required but you need to watch carefully to figure out price gaps. It is like you might need to follow multiple exchanges or trading pairs and need to compare them to find an exact and perfect arbitrage trading opportunity.
Arbitrage trading opportunity may happen between an international exchange and your local exchange or within your crypto exchange but between different pairs. Hope you might have heard like Korean exchanges were trading BTC $200 ahead due to heavy local demand. This could be one real time example to understand arbitrage trading.
Correct, if arbitrage is done correctly it is as if you could print money by yourself but the profits in each trade will be very small which is why you need big capital to even consider that form of trading, however there are many possible pitfalls that people can make and instead of earning profits they could lose money, one of the most obvious is that an exchange could have a BTC/USD pair exchanging for an higher price but that is because there are fears that the exchange could disappear as they disallowed fiat withdrawals and everyone is accepting a small loss to buy bitcoin just to get out of that exchange.