>as soon as somebody somewhere makes a loan using bitcoin, then bitcoin becomes a fractional reserve currency.

No, there is the other option called full reserve banking.

i don't get how you can loan bitcoins in any other way other than full reserve - unless you somehow put trust into the bank (which you have to for fiat money, but for bitcoins, you can verify and so don't have to trust credit/notes offered by the lender).

Because somebody will get the idea to offer of interest on bitcoins in exchange for lending their coins. And somebody will take that bet.

It's not like cryptocurrencies can change human nature.

interest bearing loan has nothing to do with the "reserve" you need.

if i borrowed 1 bitcoin from you, and then i default, then how are you going to pay back that bitcoin to whoever deposited it?

Same way you don't get back the exact serial-numbered dollars you loan (deposit) to a bank. It's not the _identity_ of the bitcoin which matters, it's the value.

If you want a lockbox, get a lockbox. If you want a deposit account, you're storing and retrieving fully fungible and interchangeable entities.

From the profits earned off of other borrowers. Like banks do.

And it's worth remembering that banks aren't the only institutions that make loans. There are a lot of businesses that extend credit (post-paid services, invoicing with net 60 terms, etc), which has the same exact macroeconomic effect.