They have an agreement with 3rd party custodians for checks and balances outside of a few of them.

"Most spot Bitcoin ETFs rely on a third-party custodian to actually store the Bitcoin they hold — much like how spot gold ETFs often keep their physical gold holdings in the vault of a third-party custodian.

Eight out of the 10 currently-trading spot Bitcoin ETFs use Coinbase (COIN) as their Bitcoin custodian. The only exceptions are the Fidelity Wise Origin Bitcoin Fund (FBTC), which uses Fidelity itself as a custodian, and the VanEck Bitcoin Trust (HODL), which uses Gemini. "

source - https://www.nerdwallet.com/article/investing/spot-bitcoin-etf#:~:text=Most%20spot%20Bitcoin%20ETFs%20rely,COIN)%20as%20their%20Bitcoin%20custodian.

Next this article jumps in the the APs and Custodian relationships:

"When comparing the ‘In Kind’ and ‘In Cash’ redemption models for ETFs, various differences come to light. In the "In-Kind Redemption" model, APs can redeem ETF shares by directly receiving the underlying assets, typically Bitcoin, instead of cash. This process involves transferring a matching portfolio of securities to the ETF issuer, who then creates new ETF shares based on this portfolio, thereby avoiding immediate selling for cash. The benefits of this model include tax efficiency, liquidity maintenance, avoidance of market impact, creation/redemption flexibility, and transparency. However, considerations such as logistics, custodial challenges, and feasibility during extreme market conditions must be acknowledged."

source - https://www.coinbureau.com/education/how-bitcoin-etfs-work/