How to enter @ and € on a German MacOS installation?
It’s alt+L for the @ and alt+E for the €.
https://support.apple.com/de-de/HT201794
If on the Mac you set the Input Source to "German Standard" (and not "German"), then you will have @ produced by Option/alt Q and € by Option/alt E. That may be how your 3rd party keyboard is printed.
The layout shown in HT201794 reflects an Apple keyboard which corresponds to the "German" input source. The newest Apple keyboards correspond to German Standard.