How to adjust left sidebar menu to dropdown with arrows

Well on default there are no arrows – aren’t they. – and it is no toggler – it reacts on hover – not on click.
You can have an arrow on those who have a submenu. f.e.:

#avia-menu .menu-item.dropdown_ul_available:before{
    position: absolute;
    top: 12px;
    left: -15px;
    content: "\e883";
    font: 20px/1 entypo-fontello;
    speak: never;
    display: block;
    -webkit-font-smoothing: antialiased;
    -moz-osx-font-smoothing: grayscale;
    text-decoration: none;
    z-index: 1;
    transition: all 0.7s ease;
}

#avia-menu .sub-menu .menu-item-has-children:before{
    position: absolute;
    top: 12px;
    right: 10px;
    content: "\e883";
    font: 20px/1 entypo-fontello;
    speak: never;
    display: block;
    -webkit-font-smoothing: antialiased;
    -moz-osx-font-smoothing: grayscale;
    text-decoration: none;
    z-index: 1;
    transition: all 0.7s ease;
}

#avia-menu li.menu-item.dropdown_ul_available:hover:before{
    transform: rotate(-90deg);
    color: red !important;
    transition: all 0.7s ease;
}

#avia-menu .sub-menu .menu-item-has-children:hover:before{
    transform: rotate(-90deg);
    color: red !important;
    transition: all 0.7s ease;
}

see:

What number did you use for your layout?
https://iks-menu.ru/previews/

on most of those previews : they work with flex model.
If you gave to the first flex-item an order number 2 they change position:

.iksm-terms .iksm-term__link {
    order: 2;
}

to bring the parents in line with the items with children you can do that:

.iksm-term:not(.iksm-term--has-children) {
    margin-left: 40px;
}

or style the menu 5 to your needs ( different arrows, and background)