This is my script, put in the last column this information:

1. Insert into the Cell (In the last cell = lastcolumn for the row that you modify) - Date in GMT -3, if you want to put the same in other TZ just modify the -3 for you concurrent timezone.
If you want to put the date automatically in your default TZ, just modify this:

var date = Utilities.formatDate(new Date(), "GMT-3", "dd/MM/yyyy HH:mm");
to
var date = Utilities.formatDate(new Date());

2. Insert a Comment with the Cell+User who modify it.
3. Appear a popup when you update something. Only appear to the ActiveUser.

function onEdit(event)
{
  var ss = SpreadsheetApp.getActiveSpreadsheet();
  var actSht = event.source.getActiveSheet();
  var actRng = event.source.getActiveRange();
  
  var index = actRng.getRowIndex();
  var dateCol = actSht.getLastColumn();
  var lastCell = actSht.getRange(index,dateCol);
  var date = Utilities.formatDate(new Date(), "GMT-3", "dd/MM/yyyy HH:mm");
  
  lastCell.setValue(date);
  lastCell.setComment("Ultima Modificacion: " + actRng.getA1Notation() +' por '+Session.getActiveUser());
  ss.toast('Ultima Modificacion = '+ actRng.getA1Notation()+' por '+Session.getActiveUser());
  }