Hello,

First you need to define a function according to your requirement, In the function you need to get the orderlines of corresponding _order,

ie,

order_lines = self.env['sale.order.line'].search([('order_id', '=', self.id)])

now you need to get the products and corresponding quantity, for that

product = []
qty = []
for order in order_lines:
    if order.product_id not in product:
        product.append(order.product_id)
        qty.append(order.product_uom_qty)
    else:
        index = product.index(order.product_id)
        qty[index] += order.product_uom_qty

Here you will get each product and their qty respectively in the lists product and qty. You can add the values from the list to the needed field by looping them.

Regards