In modern bitcoin, what is the role of an exchange? Do most people hold their own wallet? If you have an offline wallet, what's the work involved with using t

In modern bitcoin, what is the role of an exchange? Do most people hold their own wallet? If you have an offline wallet, what's the work involved with using that cold key to perform a transaction? And for an organization like Tesla, do they now have 2 FTE to manage their holdings or do they just have 3000 wallets with different amounts of BTC some of which are online and some of which are offline? This all seems like an enormous PITA to actually use for 99.9% of human sized transactions; an exchange seems like a reasonable mitigation against that, but now you're 100% at the mercy of something that's as trustworthy as an 1830's bank.