Grid tells boxes where to go. Flex tells content how to take up space. Technically the window is a box with content, and technically your content can be split

Grid tells boxes where to go. Flex tells content how to take up space. Technically the window is a box with content, and technically your content can be split into a grid of boxes, so grid and flex can kind of do each other's jobs if you force them. But if your content isn't really boxes or your grid is really content, consider using the other.