repeater ID How it work?

You can use the onItemReady of the repeater with the $item scope…

#repeater1 - repeater
#button1 - button in the repeater

$w.onReady(function () {
 $w("#repeater1").onItemReady( ($item, itemData, index) => {
  $item('#button1').onClick((event) => {
   console.log(index);
  });
 });
});