If the ETF is listed, you just buy it through a broker on the exchange like any other stock. To get your money out, you would sell for fiat on the market. It has nothing to do with any BTC in a wallet where you would continue to control the keys, or BTC that you would use for personal transactions. You wouldn't be buying BTC. You would be purchasing a security - same as how the gold & silver ETFs are structured. The ETF is not the same as owning BTC. It is a derivative that tracks the price movements of BTC.