UK Keyboard
[see below for other languages]
Cmd ⌘ `
Cmd ⌘ Shift ⇧ ` to go the other way.
Left of z on a UK keyboard [non-shifted ~ ]
Note: This only works if all windows are in the same Space, not if they are spread over multiple Spaces, which includes fullscreen.
BTW, specifically in Chrome & Safari, but no other app I know of on Mac, Cmd ⌘ (number) will select individual tabs on the frontmost window.
It also would appear that Cmd ⌘ ` is yet another of those language-specific shortcuts; so if anyone finds any more variants, please specify for which language & keyboard type.
If anybody finds new combos for different languages, please check Keyboard layout here - Apple KB: How to identify keyboard localizations - & add that as well as which Input Source you use in System Prefs > Keyboard > Input Sources.
Add a keyboard picture from the KB page too, if that would help.
That will make it easier for future Googlers.
Further info:
You can change the keys in System Prefs > Keyboard > Shortcuts > Keyboard...
though it doesn't list the reverse direction, it does still work when you add shift to that new combo. I tested by moving mine from ` (and ~ ) to § (and ± )