cmd + ` switches between windows of an app. Unfortunately, that's the most awkward key combo imaginable on non-US keyboards. Still better than having to mouse down to the (hidden) dock, but only marginally.

GNOME does it right, and uses super + <the key above tab>. Works the same as the Mac in the US, but is infinitely better in the rest of the world.

(you might be able to remap it on macos using an undocumented 'hidutil' command, but I've never got it to work on an external keyboard)