All models of the Apple Watch include the optical heart rate sensor array needed to measure heart rate both manually and passively.
The electrical heart sensor necessary to perform ECG recordings is available on Apple Watch Series 4 and later but not on Apple Watch SE. Blood oxygen readings are supported on Apple Watch Series 6 and later, excluding Apple Watch SE. Fall Detection is not available on Apple Watch Series 3 and earlier.
The ECG app is only available on Apple Watch Series 4 and later in select countries and regions. It is not available on the Apple Watch SE. Also, it is not intended for use by people under 22 years old.