Maximize window shortcut
One of the default settings in System Preferences > Dock is [√] Double-click a window's title bar to zoom; however by default, Finder does not have a keyboard shortcut to zoom a window.

One may add a keyboard shortcut for the Finder > Window > Zoom command in System Preferences > Keyboard > Shortcuts > App Shortcuts, as shown in the image below.

Click the [+] and fill in the sheet, then click the Add button: