make a layout for smallest screen, then play with the "anchor" property of objects in the inspector. Proper anchoring should stretch the layout on the larger screen.
As suggested by Torsten, the Autosizing anchors can allow you to stretch (and also keep centered) layout objects.
To automatically switch layouts depending on screen orientation you can assign a script trigger to the layouts to be triggered by 'OnLayoutSizeChange'... The script that gets triggered can use Get(ScreenWidth) and Get(ScreenHeight) to determine which orientation has been switched to and then use Go to Layout to go to the appropriate layout.... e.g:
If [ Get(ScreenWidth) > Get(ScreenHeight) ]
Go to Layout [ “landscape” ]
Go to Layout [ “portrait” ]
awesome! Thank you