Being new to BTC I would like to know where the BTC are held that are for sale? I am learning as I can understand a complicated subject the deeper I dig into BTC. My stepson is a BTC bull, and he says I need to spend 1000 hours to understand BTC? Other than private wallets, is there a pool of BTC, and how many are there of the 21000000 total available? I am looking to buy more BTC ETF before I go deeper into buying the actual BTC wallet.