Hi, they are very pricey and I am not wealthy but this one is affordable.
I found this:
"Apple Watch Series 4 and later and all models of Apple Watch Ultra also have built-in electrodes in the Digital Crown and the back of Apple Watch, which can measure the electrical signals across your heart when used with the Heart Rate app or the ECG app."

Found a series 4 on Amaz for 150.00 but is "Renewed."
This system says I am new so cannot post links but this is the heading on this listing:

"Apple Watch Series 4 (GPS, 44MM) - Space Gray Aluminum Case with Black Sport Band (Renewed)"

Have an Fitbit that works, well give the same info as the Apple. The Apple only works with an apple phone ...