No problem @spielefritze, I’m fairly confident your English is leagues ahead of any German I might attempt 
The xone-dkms package has xone-dongle-firmware as a dependency. Therefore, installing xone-dkms will automatically install xone-dongle-firmware. Letting the system install it automatically as a dependency also ensures that the xone-dongle-firmware package is correctly marked as a dependency, making uninstallation cleaner.
So for you, to install xone-dkms and its dependencies, you’d run:
yay -S xone-dkms
General AUR helper commands:
To install an AUR package:
yay -S package-name
To uninstall an AUR package:
yay -R package-name
To uninstall an AUR package and all its dependencies and config files (use with care, with commonly depended on packages!):
yay -Rcns package-name