If we want to turn altcoin to cash, will we need to sell it for BTC first, and then send the BTC to service for conversion to USD/EURO before finally to bank?
It all depends on what altcoins you have on hand, because on some exchanges there are trading pairs for USD with some altcoins such as ETH or BNB. You can sell these altcoins directly to dollars directly on the exchange. There, on the exchange, you can withdraw cash to your credit or debit card.