Some in exchanges, some in mobile wallet and some in hardware wallet. I diversified my coins, it is safer instead of holding all in hardware wallet.
No, the safest way to store a coin is in cold storage, then keep it private and secure, if you want to save your Coin on exchange there is a possibility your account got hacked or the exchange get hacked, so make sure you use strong password and enable 2-factor authentication if you want to hold in exchange, and if you store it on a mobile wallet there is a chance your phone is missing then you can't recover your coin