What is the difference between Row and RowLayout?

Row is a Item Positioner. Positioner items are container items that manage the positions of items in a declarative user interface.

RowLayout is part of Qt Quick Layouts. They manage both the positions and the sizes of items on a declarative user interface, and are well suited for resizable user interfaces.

Your code with RowLayout should look like this:

RowLayout{
    anchors.fill: parent
    spacing: 0
    Rectangle{
        Layout.fillHeight: true
        Layout.preferredWidth: parent.width * 0.3
        color: "blue"
    }
    Rectangle{
        Layout.fillHeight: true
        Layout.fillWidth: true
        color: "red"
    }
}