If you wanted pesos to spend when you get to Mexico, you'd exchange your dollars for pesos. But if you had some obscure Asian currency and showed up in some obscure island in the Caribbean, they might not be able to swap it without you exchanging back to dollars before to go to the Caribbean.

In a lot of cases, bitcoin will be the only one you can use to exchange for a smaller one. So you sell for bitcoin and buy with bitcoin. And satoshis is just the name for a decimal portion of bitcoin.

But tbh if you need a ELI5 for that, my advice is to not get into it as the swings are insane. Also, like downvote_ferry said, "not your keys, not your coins". If you log into an account using a username and password, those are not your coins. You should always transfer to a wallet that you have the key only stored on a physical piece of paper locked up.