Check out example codes for "read every cell in a column google app script". It will help you in understanding the concepts better.

Code Example 1

function appendString() {
    var range = SpreadsheetApp.getActiveSheet().getActiveRange();
    var values = range.getValues();

    values.forEach(function(row, rowId) {
        row.forEach(function(col, colId) {
            values[rowId][colId] += " string";
        });
    });

    range.setValues(values);
}

Code Example 2

function appendString() {
  var range = SpreadsheetApp.getActiveSheet().getActiveRange();
  var values = range.getValues();

  var modified = values.map(function (row) {
    return row.map(function (col) { 
      return col + " string"; 
    }); 
  })

  range.setValues(modified);
}

Code Example 3

function appendString() {
  const range = SpreadsheetApp.getActiveSheet().getActiveRange();
  const values = range.getValues();
  const modified = values.map(row => row.map(currentValue => currentValue + " string"));
  range.setValues(modified);
}

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