futures originated from commodities trading, where you wanted to lock in the price of a ton of corn or wool or whatever because you fear the price will go up.

you don’t need them for currency arbitrage. you just need very fast connections to multiple currency markets, low transaction costs, and mispriced exchange rates.

it’s riskless because the money itself is mispriced and you can buy it for one price (in your own currency) and sell it for another all in one transaction. you get the difference for free. that’s what makes it riskless and that’s what makes it arbitrage.

keep reading those investopedia articles, it’s all in there somewhere!