The twenty-first amendment was the only to be proposed by congress and ratified state convention which process is demonstrated here?

A concurrent resolution is a resolution that is passed by both houses of Congress (the House of Representatives and the Senate) but does not require the signature of the President. Concurrent resolutions are used to express the sense of Congress on a particular issue, but they do not have the force of law.

In the case of the twenty-first amendment, Congress proposed the amendment by passing a concurrent resolution. The amendment then had to be ratified by conventions in three-quarters of the states in order to become law.