I have a CSV file with the coordinates of the vertices of my polygons (in the picture they correspond to "left", "right", "top", "bottom"). Actually is a grid of polygons. How can I import the file in QGIS as polygons and not as points?
You can do it with expressions, even loading your excel file, without converting to .csv, load your excel file as if it was a vectorial layer
Simply run this expression
geom_from_wkt( concat('POLYGON((',to_string("left")+' '+to_string("top")+',',to_string("right")+' '+to_string("top")+', ', to_string("right")+' '+to_string("bottom")+',', to_string("left")+' '+to_string("bottom"),'))
You will get a result like this:
I'm assuming you want the rectangles with diagonal (left,top) and (right,bottom) as the required polygons.
You could import the CSV as points, say x=left, y=top, and then use processing (or even expressions) which access right and bottom as attributes and transform to the desired rectangles.
However, it will be faster to preprocess within Excel (or whatever your spreadsheet program is) by combining the attributes into a WKT Polygon string, which unambiguously specifies the polygons in a standard format.
In cell G2 (or row 2 of a new column if G is used), put
="POLYGON(("&B2&" "&C2&", "&D2&" "&C2&", "&D2&" "&E2&", "&B2&" "&E2&", "&B2&" "&C2&"))" and copy it down. It generates a polygon WKT string that goes around the points clockwise. You can then import the CSV taking the geometry from this column and QGIS will interpret it as a polygon (rectangle) for each line.