Where can you pay online with the Postbank Girocard?

Amazon, Netflix etc?

Wherever it is accepted.

Nowhere. A Girocard is not suitable for such payments.

At most, it is possible to debit the current account by direct debit (if this is offered), but that has nothing to do with the card.

If you can pay there by means of an IBAN (SEPA), you have to deposit the bank data there and the payment will be processed.

A normal bank card (checking card) is not a VISA / master card, payments will not be used for this