Where can I change the "Add to cart" or in German "Zum Warenkorb hinzufügen" Text on the product page? Where is that template file and where do i need to copy it in my custom theme?
You can find out it in
vendor/magento/module-catalog/view/frontend/templates/product/view/addtocart.phtml <?php $buttonTitle = __('Add to Cart'); ?>
You can change it from your theme by extend this file.
Otherwise also you can change by Csv translate file
app/design/frontend/YourTheme/ThemePackage/i18n/[Your_Language_Code].csv "Add to Cart","um Warenkorb hinzufügen"