I can withdraw all I own and turn it into gold or pebbles if I like

You don't turn it into anything. You spend it to buy something else. You can spend it without withdrawing any kind of physical representation of the currency, too, with just plain old electronic payment systems.

The borrower provides an asset (collateral) and the bank provides an asset (savings from third parties).

Plenty of loans are made unsecured, where the borrower doesn't pledge the asset. The act of money creation through lending is the same regardless of whether it's secured or unsecured loans. And even secured loans don't change the underlying ownership and control of the collateral, unless a foreclosure happens.

That's not how central banks work. You still need collateral, which you can't pledge multiple times.

Yes, but the collateral can be the loans that they've extended, which, again, were created by creating a loan balance and a deposit balance. So they can extend a loan for $100, let the borrower spend $100, and then borrow against the original borrower's loan balance.

No, it can't be done infinitely, but I never claimed that it could be. I'm just saying that the process itself is entirely ephemeral, through written or electronic records alone.