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.
As other users mentioned, the bitcoins are stored in the blockchain, but if you want to buy them then you can use the exchanges, there is where users sell their bitcoins, if you want to buy huge amounts then you must use big exchanges with a lot or bitcoin orders.
Right now bitcoin is in a complex situation, the fact that It lost close to 10% in the past 24h will bring chaos to the markets, I would wait the crash ends before buy.