European (or at least Dutch) debit cards are not Visa or Mastercard. They are either Maestro or V Pay.

These cards don't have a 16 digit card number and CVV that you can in online shops and pay with those. This has the advantage that these numbers also cannot be "stolen" and then used by criminals to buy things.

Debit cards are really just for physical payment, where you either use contactless payment or by using a PIN. For online shopping, the shops talk to the banks by using an OAuth-like API that can be used to either pay once or set up a monthly payment. There's a few different of these APIs throughout Europe, in each country usually only 1 or 2 are used. Some examples of these APIs are: Bancontact, iDeal, Giropay and Sofort

European webshops usually only support the ones in the country that they are located in, and for all others they either support credit card or bank transfers (which are free and standardized within Europe). American webshops usually only support credit cards, so that's the most common reason for people to have one.

Ah, I see. Well, just get Visa or MasterCard, even in Germany it's very easy these days.

Errr depends on the area. Sometimes they have EC but not VISA and then you look like a cashless moron (I literally only ever have minimal 'not a vagrant' cash in France, and never use it, especially since they added touch-pay for <20EUR and increased it to <50EUR during first lockdown).

Not sure about online stores, but renting a car in the US with a debit card is a pain from experience.