My doctor wanted me watch my heart rate. Does my watch or iPad store that information, so I can show the doctor?

Not sure why everyone is telling you about the ECG results because thats not what you asked about.

Apple Watch heart rate data, as with all health related data, is stored int he Health app on your iPhone- it is not accessible via iPad. From the Health app, go to Browse, Heart, Heart Rate, then you can filter down the data by hour, day, week, month, or year. While I'm not aware of a way to export it, you can screenshot it and share it that way.

Yes.

See here for how to export your data: link-> Taking an ECG with the ECG app on Apple Watch Series 4, Series 5, or Series 6 - View & Export - Apple Support

The data is not linked to your iPad.

It is all in the health app on your iPhone.

You can export a copy of your ECG to your doctor,here is an old one of mine;