From index rules to holdings
An ETF designed to track an index generally owns the securities represented in that index in approximately the proportions specified by the index methodology.
When the index changes
Index providers periodically rebalance or update their constituents. A company may be added, removed, or assigned a different weight as the rules are applied.
Tracking is an objective, not a guarantee
An ETF will not normally match its index perfectly. Expenses, trading costs, cash, taxes, and other operations can create a difference between fund and index performance.