What are the coordinates (XYZM) for grids in QGIS? Or how do I add columns in the QGIS field calculator?

When you create a grid in QGIS, in the attribute table for each square, you have a corresponding column left, right, top, bottom in coordinates. Which of these four correspond to the X, Y, Z, M coordinates?

I am trying to import a grid from a CSV file with the left, right, top, bottom columns. I had to export the grid because I couldn’t figure out how to add the values from multiple columns correctly in the field calculator in QGIS. So I exported it into a CSV, added the columns in excel, now I want to bring the fi.e back into QGIS.

Alternatively, how do I create a new column in the attribute table that adds the values in multiple columns by using the field calculator? I tried "Col1"+"Col2"+"Col3" and sum(col1,col2,col3) but couldn’t get completely correct values for some reason. In the first case it only seemed to be adding some of the columns.