The tools allowing you to select features in the visualization window are regrouped in the Selection menu:
Regardless you want to make a simple or a multiple selection, you have to be in edition of the correct construction layer to target an object.
You have two ways to select a unique feature:
The multiple selection is made with the selection tool. You can either click and drag your cursor to select all the features within the rectangle or by selecting single features one by one maintaining the Shift key.