How do I use Apple Cash card in Germany

Depends on what you mean exactly by "use".

If you have it already set up, you can use it in stores and other merchants that support Apple Pay by selecting it when paying for something from your available cards and payment methods.

If you mean you want to set it up as a German citizen, then that is not possible, as it's only available to US citizens with a US physical address at this time.

click here ➜ Set up Apple Cash - Apple Support

Apple Cash is a Visa and accepted worldwide at merchants that accept Apple Pay and Visa.