switch between windows of the same app on Mac

Command + Tilde

I don't know. Did you try pressing the command key plus the key to the left of the one key? If that doesn't work someone else will have to answer.

On a U.S. keyboard, it's the key to the left of the one key.

Check System Preferences > Keyboard > Shortcuts > Keyboard.

The following key (red box) combination is the German keyboard equivalent of the U.S. ⌘+` (back tic).