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solution 1.) (only for computer, for mobile display, you still need to add custom.js)

./themes/classic/assets/css/custom.css

.product-add-to-cart > .control-label, 
.product-add-to-cart > .product-quantity > .qty,
.product-line-grid-right .input-group
{
    display: none !important;
}

solution 2.) (work in computer and mobile)

find 1: (/themes/classic/templates/catalog/_partials/product-add-to-cart.tpl)

<span class="control-label">{l s='Quantity' d='Shop.Theme.Catalog'}</span>

change to:

<span class="control-label" style="display: none !important;">{l s='Quantity' d='Shop.Theme.Catalog'}</span>

 find 2:

<div class="qty">

change to:

<div class="qty" style="display: none !important;">

find 3: (/themes/classic/templates/checkout/_partials/cart-detailed-product-line.tpl)

            {if !empty($product.is_gift)}
              <span class="gift-quantity">{$product.quantity}</span>
            {else}
              <input
                class="js-cart-line-product-quantity"
                data-down-url="{$product.down_quantity_url}"
                data-up-url="{$product.up_quantity_url}"
                data-update-url="{$product.update_quantity_url}"
                data-product-id="{$product.id_product}"
                type="number"
                inputmode="numeric"
                pattern="[0-9]*"
                {if $product.minimal_quantity > 1 || $product.quantity_wanted}
                  value="{$product.quantity}"
                  min="{$product.minimal_quantity}"
                  step="{$product.minimal_quantity}"
                  data-step="{$product.minimal_quantity}"
                {else}
                  value="{$product.quantity}"
                  min="1"
                {/if}
                name="product-quantity-spin"
                aria-label="{l s='%productName% product quantity field' sprintf=['%productName%' => $product.name] d='Shop.Theme.Checkout'}"
              />
            {/if}

change to:

<span class="gift-quantity">{$product.quantity}</span>