In Google Sheets (and Excel or Mac Numbers should be similar for most of this), you normally enter a cell's value by hitting enter which moves to the next cell. But if you need multiple lines in a single cell, you need to know the trick of holding command/Apple-key when pressing enter.
If you want to import a CSV file into Google Sheets but get the same effect, then you need to escape the new-line markers.
First, a cell which contains more than one line (instead of each line being on a new row) needs to be "escaped" with double quotes at the start and end of the cell's...