Yes you are absolutely correct.
You cannot add or delete elements at any breakpoint. You can only display:none; or display:block, in line... Etc...
The only way to achieve this is to have the element in both rows and display:none; the element in the second row until you get to the breakpoint you want it to show. Then display:block; to it and display:none to the element in the first row.
You are correct about the breakpoints. They do add media queries at those breakpoints and you can adjust css stylings at each point. No you cannot add/replace/delete elements at any breakpoint without deleting them from the project.
https://www.youtube.com/channel/UCL8qVv … ttneYaMSJA