Apple Watch Series4

Series 3 can monitor heart rate, provided you don’t have anything like wrist tattoos which interfere with the optical sensor. It is not continuous monitoring, you will get spot checked at intervals, on my Series 3 it seems to sample about every 5 minutes if I leave it to do its own thing. You can also initiate an extra check manually outside the automatic sequence.

Apple Watch is approved as a pre-diagnostic screening level device for AF, but it sounds like you already know about a positive AF diagnosis so that function will not tell you anything new. The heart rate monitor will still be useful to you within its measuring range (30-210 per minute) but if the AF anomalies go outside that range it will have reduced accuracy.

Probably, but clinical use and medication control are not an approved use for the heart rate monitor in Apple Watch. I suggest you discuss your plan with your cardiologist before you make the final decision, and give due consideration to their comments.

It's pretty accurate or it would not have been approved. Here is Apple's explanation of how it functions...https://www.apple.com/apple-watch-series-4/health/

My pleasure.