First, congrats on starting your first landing page.
Straight to your question… The short answer is that you need to utilize the Page Sections feature when you are building your landing page.
Now the longer explanation: When building a landing page, each element is placed in to the most appropriate section. As long as your elements do not move between sections, they’ll keep their relative placement in both Desktop & Mobile view.
Only when you try to move an element from one section to another would you also move into the other view (Desktop or Mobile respectively).
It also helps to keep in mind that if you place an element, for example a text element, inside a box element in your Desktop view, the mobile view also needs these two elements to go together.
Otherwise, if you try to take out the text element out from the box element only for your Mobile view, you’ll end up moving it in the Desktop view as well.
Last but not least, if you really need to move elements between sections, you can do so by holding down the Command/Contorl key on your keyboard.
This is the official documentation regarding the mobile editing.
Hope this points you in the right direction.