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 (
    74537, 74536, 74535, 74534, 74533, 74532, 
    74531, 74530, 74529, 74528, 74527, 
    74526, 74525, 74524, 74523, 74522, 
    74521, 74520, 74519, 74518
  ) 
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 20 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
74518 6M
74519 6M
74520 6M
74521 6M
74522 6M
74523 6M
74524 6M
74525 6M
74526 6M
74527 6M
74528 6M
74529 6M
74530 6M
74531 6M
74532 6M
74533 6M
74534 6M
74535 6M
74536 6M
74537 6M