How do I download the ecg app?

MichelPM wrote:

No other Watch series supports the ECG feature/function.

Apple watch 4 and above feature the ECG app.

Quote from apples support document;

How to use the ECG app

The ECG app can record your heartbeat and rhythm using the electrical heart sensor on Apple Watch Series 4, Series 5, or Series 6* and then check the recording for atrial fibrillation (AFib), a form of irregular rhythm.

If you own a Watch series 6, the ECG feature is a series 6 exclusive feature/function already part of watchOS.

There is no app to download.

No other Watch series supports the ECG feature/function.