Cold Wallet

Yall how does this work?? I get not to use robinhood because then you don't own it, because you don't own the keys, which are just passwords to use the coin? I am trying to read up on how to buy, use, sell btc but the more comments and threads I read and I am more confused. Privacy, btc public? getting hacked, wallets, spreadsheets, why the wallets cold??? But buying it on robinhood... isn't buying it??? Coinbase?? Blue wallets?? Trading fee??