To add a new row in empty section click "+ Add Row with Column" button. To add new row after another row, take a look top center of the existing row. Find the following icon and click on it.
Then click on pre-configured columns to insert the row with column.
In an empty row, Click on "+ Add Columns" to add new column. To add new column after existing column, Find 3 dots on column controls and click on it then click "New Column". Click on column size to insert it. See the video above for live work example.