key_len tells you how much of a composite index was used

EXPLAIN saying an index was used does not mean all of it was, and the column that says how much is the one nobody reads.

CREATE INDEX idx ON orders (status, customer_id, placed_at);

EXPLAIN SELECT * FROM orders
WHERE status = 'paid' AND placed_at > '2019-01-01';
-- key: idx    key_len: 62

EXPLAIN SELECT * FROM orders
WHERE status = 'paid' AND customer_id = 12;
-- key: idx    key_len: 70      ← two columns, not one

The number is bytes, so working out which columns it covers means adding up their storage sizes — a VARCHAR(20) in utf8mb4 that is nullable is 20*4+2+1. Tedious, and it settles the argument about whether an index is actually helping. A range condition stops the index being usable for anything after it, which is why the first query only uses the leading column despite naming two.