The Template Builder's mobile layout is a fixed system and is pretty simple once you get used to it.
When desktop is converted to mobile it will stack each row, then stack the next row, progressing through your email.
With the new "do not stack on mobile" control, a row can be fixed, so it displays the same on desktop and mobile.
To access the "do not stack on mobile" setting, hover and click on a "Structure" block within the template.
The "do not stack on mobile" setting will appear in the "Structure" properties menu on the right.