Ecg

Yes, it is. You have nothing to download. Press the Digital Crown and scroll down to ECG if you are using the List view, or look for a white icon with a red sine wave if you are using the Grid view.

Edit: Note that the ECG is not available in all countries. The list is here: https://www.apple.com/watchos/feature-availability/#branded-ecg.

Pamela.P wrote:

Is the ECG app built into my Apple Watch 6 or do I have to download one from the App Store?

It is installed when you go through the ECG set up in Health:

Taking an ECG with the ECG app on Apple Watch Series 4, Series 5, or Series 6 - Apple Support

Note that, depending on your country or region, that feature may not be available to you.

IdrisSeabright wrote:

It is installed when you go through the ECG set up in Health:

My bad. It's been so long I have it that I completely forgot about that step.

It's a kind of weird thing, isn't it?