在响应式设计中移动顶部到底部的元素的最佳方法是什么

I have the following HTML format, what is the efficient way to position the given element on the top on a desktop and bottom on mobile devices (width < 640pixels)? Since there are lots of different devices, I don't want to write the position coordinates since the contents of the page's height varies. Any suggestions?I have the following HTML format, what is the e