No buildup. Let's get to the point.

You have a grid container, and it has some child elements divided into as many columns as you want. For now, let's say the grid container has seven elements, and those elements are divided into three columns. You want the seventh element to take up the full width of the parent.

Here's the code snippet.

.seven {
    grid-column: 1 / span 3;
}

Checkout a full example here: https://codepen.io/ccgaejza-the-selector/pen/dyELmRE