Does anybody leave a percentage of their BTC on the exchanges, or do you guys store all your BTC in your wallets, etc?
0% at exchanges. If you look at the history of exchanges, they are not safe for long term storage. And if anyone - like Celsius, FTX etc - is offering something that appears too good to be true or wants to have custody of your coins, it probably is and they are probably scammers.
If you don't have the private keys, you are merely a creditor as many have found out to their regret.