As I am currently on my mobile, I am trying to remember the keyboard combination. It's stored in muscle memory, but I believe it is <ctrl> <tab>. But it might be <alt> <tab>.

As said, not sure which of the two it is.

Check out this app. I believe it is the utility you're after...

https://manytricks.com/witch/

I use https://bahoom.com/hyperswitch to see the windows

It's <Command> + the key that's to the right of the shift key. On the German Layout that's where angle brackets are.

Not sure if this is what you are after but you can use `cmd` + `~` to switch between windows of the same app.