SELECT 
  cscart_products_categories.product_id, 
  GROUP_CONCAT(
    IF(
      cscart_products_categories.link_type = "M", 
      CONCAT(
        cscart_products_categories.category_id, 
        "M"
      ), 
      cscart_products_categories.category_id
    )
  ) AS category_ids 
FROM 
  cscart_products_categories 
  INNER JOIN cscart_categories ON cscart_categories.category_id = cscart_products_categories.category_id 
  AND cscart_categories.storefront_id IN (0, 1) 
  AND (
    cscart_categories.usergroup_ids = '' 
    OR FIND_IN_SET(
      0, cscart_categories.usergroup_ids
    ) 
    OR FIND_IN_SET(
      1, cscart_categories.usergroup_ids
    )
  ) 
  AND cscart_categories.status IN ('A', 'H') 
WHERE 
  cscart_products_categories.product_id IN (
    103742, 103741, 103740, 103739, 103738, 
    103737, 103736, 103735, 103734, 103733, 
    103732, 103731, 103730, 103729, 103728, 
    103727, 103717, 103716, 103715, 103714
  ) 
GROUP BY 
  cscart_products_categories.product_id

Query time 0.00035

Explain
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE cscart_products_categories range PRIMARY,pt pt 3 21 Using where; Using index
1 SIMPLE cscart_categories eq_ref PRIMARY,c_status,p_category_id PRIMARY 3 toolboxdigitalshop_dev.cscart_products_categories.category_id 1 Using where

Result

product_id category_ids
103714 4M
103715 4M
103716 4M
103717 18,13M
103727 4M
103728 4M
103729 4M
103730 4M
103731 4M
103732 4M
103733 4M
103734 4M
103735 4M
103736 4M
103737 4M
103738 4M
103739 4M
103740 4M
103741 4M
103742 4M