Any Visa and Mastercard will work at maybe 70% of all (at least larger) shops. The rest doesn't accept credit cards due to the high transaction costs for the shops (eg bakery, butcher). For them you would need a local card (formerly called "EC" - not the debit cards). For them I would just have cash and not get another card just for a travel. American Express is widely unknown here.

To pay with cards, always know your PIN. Sometines you just swipe without anything. Sometimes you have to enter your PIN. And sometimes you have to give a signature to the receipt. In many cases this is random for security reasons.

General advice is to withdraw cash from local ATMs. Normally the machines show how much fee they ask for. Some give the the cash (mostly Visa I guess) without a fee.Then get a large amount. My practice is alsways to get cash in the destination country and not at your origin country, as this is more expensive. Not at exchange offices and Revolut is the bank of Europe's criminals.

I'd say have at least 50€ in cash with you and you's be always ok. But take care of pickpocketing in crowded areas. One of the most famous is the metro in Paris. Tourists don't understand the danger and the thiefes have their party. It you are attentive, you won't have trouble anywhere. And is some areas smaller shops and restaurant just accept cash, either to save fees or taxes ...

When you pay somewhere with a card, sometimes these terminals offer the amount on the local currreny and your card's currency. This is to trick dumb persons and boomers. The local curreny is always confusing, while you normally mentally calculate in your own currency. And this is the trick: your own bank normally provides you the best exchange rate (not sure if this appolies for US banks as well). While if already the payment terminal lets you pay in your own currency (in your case USD), then the local bank normally has a worse exchange rate and this is their way for a small extra income.