MariaDB - Query to find unused indexes

Question

I need to find unused index on a MariaDB database and do not have 'userstat' enabled, I found this query which is supposed to find unused indexes, but I am unable to get a full understanding of this as I am just a beginner, I have an understanding of joins, subqueries, informationschema, but I am not sure how this query is calculating if the index is unused, if you can explain on what is going on in this query that would be really helpful thank you. MariaDB 10.3.28

  SELECT
  t.TABLE_SCHEMA,
  t.TABLE_NAME,
  s.INDEX_NAME,
  s.COLUMN_NAME,
  s.SEQ_IN_INDEX,
  ( SELECT MAX(SEQ_IN_INDEX)
    FROM INFORMATION_SCHEMA.STATISTICS s2
    WHERE s.TABLE_SCHEMA = s2.TABLE_SCHEMA
    AND s.TABLE_NAME = s2.TABLE_NAME
    AND s.INDEX_NAME = s2.INDEX_NAME
  ) AS `COLS_IN_INDEX`,
  s.CARDINALITY AS `CARD`,
  t.TABLE_ROWS AS `ROWS`,
  ROUND(((s.CARDINALITY / IFNULL(t.TABLE_ROWS, 0.01)) * 100), 2) AS `SEL %`
FROM INFORMATION_SCHEMA.STATISTICS s
  INNER JOIN INFORMATION_SCHEMA.TABLES t
    ON s.TABLE_SCHEMA = t.TABLE_SCHEMA
    AND s.TABLE_NAME = t.TABLE_NAME
WHERE t.TABLE_SCHEMA != 'mysql'
AND t.TABLE_ROWS > 10
AND s.CARDINALITY IS NOT NULL
AND (s.CARDINALITY / IFNULL(t.TABLE_ROWS, 0.01)) < 1.00
ORDER BY `SEL %`, TABLE_SCHEMA, TABLE_NAME;