An ETF is a basket. Its fair value, the net asset value (NAV), is what the shares inside it are worth. If the ETF trades away from NAV by more than your costs, you trade one against the other and lock in the gap.

• NAV = Σ (shares per unit × stock price).
• ETF ask below NAV − cost → buy ETF, sell the stocks. Profit = NAV − ask − cost.
• ETF bid above NAV + cost → sell ETF, buy the stocks. Profit = bid − NAV − cost.
• Otherwise no trade.