How does tax work with crypto Debit cards?
Is there a specific rule on this in the EU?

According to https://nomoretax.eu/bitcoin-taxation-developed-countries/

The fiscal treatment of Bitcoin for tax purposes differs depending on EU country.



how do I even know or calculate my taxes?


By consulting material published by the tax authorities in the country in which you are resident (and US as well if a US citizen resident outside the US).
By consulting a tax adviser.
By contacting the local tax authorities and asking them for advice. (In my country at least, they are friendly and helpful to people seeking advice).