I still can't understand who ever thought this was a good idea...
Cmd+Tab to the app, then press ↓ to choose a window from the app. Arrange using the basic OS window manager or your favorite 3rd-party window manager.
Most of the time, people have 2-3 work windows that they just want to swap between quickly, regardless of what "app" they happen belong to. The Windows alt-tab behavior captures that beautifully.
1. command-tab to desired app, keep command held down
2. hit down-arrow to enter an app's windows, you can let go of command now
3. arrow keys + return, or mouse-click, to pick window; some apps may do weird things here - preview starts with no highlights and has a list of recent documents at the bottom, for instance
This is kind of poorly documented and not very discoverable, I couldn't find it in Help>MacOS Help or Help>Tips for Your Mac, I ended up learning it from the online version of the manual.
Alternatively, and what I mostly do:
1. hit the Mission Control key (three little rectangles, usually f3), or 3-fingered swipe up on the trackpad
2. click on the desired window, keyboard is ignored here
(command-Mission Control shows the desktop, control-Mission Control shows only the current app's windows)
Even more alternatively:
1. right/command-click on an app's Dock icon
2. there may be a text list of open windows and/or recent documents, there will always be a Show All Windows entry at the bottom, keyboard nav works here
And yes to switch between windows of the same application it's Cmd+Shift+´. Not the most intuitive, but I feel it should not be combined, because that would mess up the sequence massively. If I want to switch Terminal windows, I don't want to do Cmd+Tab and then manually look through all 15 window icons to select.
Like, IMO the status quo is perfect. Sure, you gotta get used to Cmd+Shift+´, but try to do it for a week, and then it's perfectly natural.