For

<For each={list}>{(item, index) => row}</For> renders one row per item and keeps each row alive while its item stays in the list. Without key, rows are matched by item reference: item is the plain value and index is an accessor. With key={(item) => id}, rows are matched by that key: item is an accessor, so a new object with the same key updates the existing row.

Details and large-list behavior are in Lists and keys.