RE:RE:Security of the Bitcoin on these ETFsyou would have to do a minimum of research like any other investment, right? Purpose isn't a fly by night etf maker, Seif is well known in the industry. However, only the prospectus will go into details about risk of loss etc. equity etfs use custodians who hold assets and cash for them in trust (such as a big bank or bny mellon).
But given the nature (or absence) of the "asset" , it will be a different setup. you can call purpose or read their faqs.