[stdohm]

Something I use all the time to handle applications which might have multiple instances opened at once. For example, web browsers.

Press CMD-TAB to bring up the list of opened applications and then pressing UP or DOWN over a selected application to show the opened instances. After that use LEFT or RIGHT to select the instance you want to show.

[aequitas]

You can also use cmd-` (the `/~ button between shift and z on my keyboard, left of 1 on others) to switch between windows of the currently activated application.

[randomluck040]

There is an open source application called „AltTab“ that basically brings the behaviour of Windows‘ app switcher to macOS

[robertoandred]

Opened windows, not instances. There's only one instance of the application running.