The ECG on the Apple watches works by you touching the button/wheel , it makes for the equivalent of a 1 lead ecg. In hospitals it’s normally 8 leads.
It’s designed to pick up AF but it won’t tell you if you’ve had a heart attack or you’re experiencing one.
And naturally it’s got enough warnings to sink a horse.
It’s clever stuff though. Just don’t try the ECG thing if you’ve been exercising. If you’re heart rate is too high, it won’t measure it and returns effectively a void result. Sadly the wording used suggests it can’t tell if you’re OK or not, it would be better just to say pulse too high.
It depends on how hard a bike ride it was. The more vigorous the ride the more glycogen you will use up, the more strain it puts on your body that it needs to recover from. You should be able to see what your heart rate was during exercise --if it was close to your maximum heart rate for a lot of it, it’s going to take longer to return to normal.