Get the quantity of sold products on a specific day, keeping all products
If you do a ddl you get quicker a good result.
you have to LEFT JOIN the tables to get the Rows with no partners.
Also add the where clause directly to the ON Clause or else you loose the gained rows.


CREATE TABLE products (
  `id` INTEGER Primary Key,
  `name` VARCHAR(6),
  `deleted_at` VARCHAR(4)
);

INSERT INTO products
  (`id`, `name`, `deleted_at`)
VALUES
  ('2', 'Apple', 'null'),
  ('5', 'Banana', 'null'),
  ('6', 'Pear', 'null');



CREATE TABLE Orders (
  `id` INTEGER PRIMARY KEY,
  `sale_date` DATE
);

INSERT INTO Orders
  (`id`, `sale_date`)
VALUES
  ('1', '2021-02-12'),
  ('2', '2021-02-12');

CREATE TABLE ProductOrders (
  `order_id` INTEGER,
  `product_id` VARCHAR(18),
  `quantity` INTEGER,
  `price` DECIMAL(10,2)
);

INSERT INTO ProductOrders
  (`order_id`, `product_id`, `quantity`, `price`)
VALUES
  ('1', '2', '2', '2.50'),
  ('1', '5', '3', '3.50'),
  ('2', '2', '1', '2.00'),
  ('2', '5', '3', '3.50');




SELECT
    p.id AS prod_id,
    p.name AS prod_name,
    sum(po.quantity) AS quantity_sold
FROM products p
    LEFT JOIN (ProductOrders AS po 
    INNER JOIN (SELECT * FROM Orders WHERE sale_date = '2021-02-12') AS o ON po.order_id  = o.id)
    ON p.id = po.product_id
GROUP BY p.id, p.name,po.price
ORDER BY p.name;


prod_id | prod_name | quantity_sold
------: | :-------- | ------------:
      2 | Apple     |             1
      2 | Apple     |             2
      5 | Banana    |             6
      6 | Pear      |          null




SELECT
    p.id AS prod_id,
    p.name AS prod_name,
    sum(po.quantity) AS quantity_sold
FROM products p
    LEFT JOIN (ProductOrders AS po 
    INNER JOIN (SELECT * FROM Orders WHERE sale_date = '2021-02-13') AS o ON po.order_id  = o.id)
    ON p.id = po.product_id
GROUP BY p.id, p.name,po.price
ORDER BY p.name;


prod_id | prod_name | quantity_sold
------: | :-------- | ------------:
      2 | Apple     |          null
      5 | Banana    |          null
      6 | Pear      |          null


db&lt;&gt;fiddle here