I dug in the docs a little and:

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