dynamicl_control_DOM_by_event According to debug find that we can get the element by click event and so on.

dynamicl_control_DOM_by_event.ts

  setCssDynamical($event) {
    console.log($event);
    $event.source.trigger.nativeElement.childNodes[0].setAttribute("style", "font-size: 20px;font-weight: bold")
    //mat-select-trigger
    //mat-select-placeholder
    //let elt = document.getElementsByClassName("mat-select-placeholder").item(0);
    //elt.setAttribute("style", "font-size: 20px;font-weight: bold");
  }