Row VS Aligned modules
The question : shall I use a Row or a Aligned module block ? What is the difference ?
Aligned modules / widgets
This is a block that contains up to 5 modules aligned horizontally. To see how to manage the number of modules, see the documentation about the Aligned modules layout.
In this example you will have 5 modules, each module have its own module position / sidebar name.
If you publish a module / widget in each position then the 5 columns will be shown.
If you publish only 2 modules / widgets then only these 2 columns will be shown. They will adapt automatically to take the whole width (there will be no blank space). You can manage this in the layout options of the block (see the documentation about the Aligned modules layout).
Row
This is a container with columns. In each column you can drag and drop any block from the left panel.
See the documentation about Row layout to learn how to manage the columns.
En empty column will be shown even if there is nothing in it. This is useful to create empty spaces, or to put a background image on a column. Note that you can style each column separately.