While there is a keyboard shortcut to minimise any window (⌘ CommandM), there isn't a default one to maximise any.
However, there is another way to maximise any window (other than double clicking the app's title bar) as explained in Manage app windows on Mac - Maximise or minimise app windows:
Maximise a window: Press and hold the Option key while you click the green button in the top-left corner of an app window. To return to the previous window size, Option-click the button again.
You could also go to System Settings > Keyboard > Keyboard Shortcuts > App Shortcuts and set a shortcut named "Zoom" with a custom keyboard shortcut (e.g. ⌘ Command⇧ ShiftM) assigned to all applications. (See this old thread - but still working solution - for details)