A key thing in Grid is that the order of items in the source does not matter, as long as they are children of the element that has been declared as a grid. In this example I have some content and then some ads which I have placed right at the bottom of the source. I can use Grid to visually display these in between sections of content.
I could then use Media Queries to display them elsewhere in a wider layout.
Important! For accessibility purposes the logical order should be set in the source. Do not use Grid properties to adjust the logical order of your document.