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No they simply are building financial products on top of an existing one. No different than what they do with, well everything. That is Wall Street. Creating stuff out of thin air and selling it.

I just wonder that what will they do with BTC, are they using it as a payment source in case of underlying assets are traded under BTC ETF?

So, in other words, BTC ETF is just another name of an approved fund and nothing much of fancy title than has an actual use case.