Do any Apple Watch models monitor for atrial fibrillation (AFib)?

As supplied by Apple here, all Apple Watch Models following Apple Watch Series 4, as well as all Apple Watch Ultra models, support the use of ECG and thus AFib monitoring.

For more information about AFib monitoring features on your Apple Watch, please refer back to:

Take an ECG with the ECG app on Apple Watch - Apple Support

Track your AFib History with Apple Watch - Apple Support

Heart health notifications on your Apple Watch - Apple Support

LD150 wrote:

As I said A/F monitoring can exist outside if the ECG app.

A/F is indicated by irregular rhythm, based on heart beats,

Having said that it is necessary for the SE watch user to have been previously diagnosed with A/F before turning on the monitoring and turning off irregular heart monitoring.

Diagnosis cannot be made with the SE but a crude diagnosis could be made with the ECG trace on the series and ultra watches passed to a doctor for further examination.

Anyway not all Watch offer ECG which was my point, and not in all countries even where ECG is technically provided.

While I do wonder how that will work then, refer The ECG app can record your heartbeat and rhythm using the electrical heart sensor on Apple Watch Series 4 or later and all models of Apple Watch Ultra* and then check the recording for atrial fibrillation(AFib), a form of irregular rhythm.

That is not quite correct. The SE 1- do not support ECG but do support A/F monitoring. The two functions are not interdependant.

Apple Watch SE 3 - Technical Specifications - Apple (UK)

As I said A/F monitoring can exist outside if the ECG app.

A/F is indicated by irregular rhythm, based on heart beats,