The 200px is in the CSS of the CodePen in post #10.
In my view the most important thing you need to test is whether the main table will display OK on a small smartphone which may be only 320 pixels wide. If you do not specify widths etc (such as the 200px above) and keep font size reasonably small, tables can automatically be amazingly responsive. There is discussion in this recent thread about what to do if your table content will not fit within the width of a small smartphone.
You could use rowspan and colspan instead of nesting a table, as demonstrated here: