There are two basic tools that can be used. Both have their limitations.
What the starter solutions do is use objects on the right hand and/or bottom edge of the layout to "stretch" the window out to larger window sizes. This is done with the auto-size anchors to slide or stretch objects to fit the larger layout size.
But the same starter solution also detects whether you are using an iPhone, iPad or computer to open the file and also selects a completely different layout for each group of devices. This is both to use layouts adapted for different screen sizes and also to take best advantage of whether the user is using a touch or mouse driven interface.