Ordinary investors and institutions (excluding AP) cannot directly participate in the creation or redemption of ETF units; they can only trade IBIT on the secondary market. In other words, apart from AP, other participants cannot create or redeem units.
The net inflow or outflow of BTC holdings commonly referred to in the market is not the result of secondary market transactions, but rather the change in issued units caused by AP unit creation or redemption in the primary market. An increase in issued units is a net inflow, and a decrease is a net outflow.
However, ETF secondary market trading itself does not directly result in an inflow or outflow of BTC.