yen, circle, round
圓 centers on a smooth, closed curve: a circle or something round. The yen sense is a specific application of that round shape to currency.
圓 is the traditional form of 円. It combines 囗 (enclosure) with 員, which likely contributed the sound and the idea of roundness or completeness.
The enclosure 囗 surrounds 員, like a round container holding something complete. Picture a perfectly round coin inside a circular border: 圓 is round, like a yen coin.
For エン, imagine a perfectly round yen coin: yen -> エン, and the coin's round shape matches the kanji's meaning.