Ann: Change in substantial holding, page-9

Creating and Redeeming ETFs
The important difference between an ETF and a stock is
that, unlike a stock, an ETF is open-ended. Creation and
redemption of units is facilitated by market makers who
interact with the fund in large blocks of units (typically 50,000
or 100,000 units). This mechanism facilitates the primary
market in ETFs, and works through the in-kind transfer of
a basket of securities to (or from) the fund. Thus if there is
net buying of an ETF, more units are created to meet that
demand. Similarly, if there is net selling, units are redeemed.
Units are created or redeemed at the full net asset value
per unit.

https://www.blackrock.com/ca/individual/en/literature/.../understanding-etfs-en-ca.pdf