Those would be

Control-F4 Move focus to the active (or next) window

Shift-Control-F4 Move focus to the previously active window

If you have the F-key not enabled (Enable them in system preferences

Or use the fn key in addition to the above short cut.

Those would be

Control+F4 - Move focus to the active (or next) window

Shift+Control+F4 - Move focus to the previously active window

If you have the "F keys as standard function keys" option disabled in System Preferences > Keyboard, enable it.

Or use Fn in addition to the above shortcuts.