If you really want a wrist-mounted pulse oximeter with a built-in ECG, of medical quality, there is only one product on the market currently and that is the Withings Scan Watch. However, it is quite expensive.
Withings have been in the mobile health market for many years, the company changed hands a few times [at one point it was bought by Nokia, who were then bought by Microsoft]. This is a technical problem that giants like Apple and Samsung are still struggling with, and it's good to see that this smaller French firm, who have been striving for years, are first to the market with medical-grade consumer technology.
Generally, we would recommend instead you buy a pulse oximeter, costing around £20, more info here: https://www.digitalhealthcoachuk.net/pulse-oximeters
Features of the Withings Scan Watch:
ELECTROCARDIOGRAM — Detect atrial fibrillation or normal heart rhythm, monitor low and high heart rate in just 30 seconds via ECG
WRIST-BASED OXIMETRY — Provides on-demand medical-grade oxygen saturation level (SpO2) in just 30 seconds
HEART SCANNING — Check heartbeat and receive notifications when heart rate is atypical (low or high) and if irregular rhythms are detected
HEART RATE TRACKING — Track heart rate continuously during workouts to optimize training, and monitor daily and overnight
heart rate to help improve health over time
BREATHING DISTURBANCES DETECTION — Detect breathing issues during the night and identify signs of sleep apnea
SLEEP TRACKING — Delivers a Sleep Score based on light and deep sleep cycles, sleep duration and wake-ups
24/7 ACTIVITY TRACKING — Dedicated workout metrics, Fitness Level via VO₂max estimation, connected GPS, elevation and water resistance to 50 meters