RIGHT JOIN is a LEFT JOIN written the other way round: it keeps every row from the table after the keyword. a RIGHT JOIN b returns the same rows as b LEFT JOIN a. Most people write LEFT JOIN and put the table they want to keep first, but you'll meet RIGHT JOIN in other people's queries.
FULL OUTER JOIN keeps unmatched rows from both sides. It's the tool for reconciling two lists: what is in the first, the second, or both.
SELECT COALESCE(a.customer_id, b.customer_id) AS customer_id, a.orders, b.tickets
FROM march_orders AS a
FULL OUTER JOIN march_tickets AS b ON a.customer_id = b.customer_id;
Either side can be NULL, so take the key with COALESCE(a.key, b.key). MySQL has no FULL OUTER JOIN; there you combine a LEFT JOIN and a RIGHT JOIN with UNION.