F-Droid is generally a good source for open-source Android Apps. There are 2 more Apps, maybe they're less finnicky. But they haven't been updated in 2 years and I haven't tested them. You can install the F-Droid store and just try all of them. I haven't really played with stuff like that since I got my MiBand fitness band. (Which isn't really precise with heart rate but this is a different story.)
Bunch of stuff here, but it looks like the Apple Watch implementations may be limited:
For Android: https://github.com/Harishwarrior/pulse_rate_monitor
There are more, older ones in the F-Droid repository.
This does seem to work, but I will say that it is slightly finnicky, as compared to an app like what I mentioned in my post.
https://search.f-droid.org/?q=heart+rate
F-Droid is generally a good source for open-source Android Apps. There are 2 more Apps, maybe they're less finnicky. But they haven't been updated in 2 years and I haven't tested them. You can install the F-Droid store and just try all of them. I haven't really played with stuff like that since I got my MiBand fitness band. (Which isn't really precise with heart rate but this is a different story.)
Bunch of stuff here, but it looks like the Apple Watch implementations may be limited:
https://github.com/topics/heart-rate
I should say too, the fingertip pulse oximeters they use in doctors offices and hospitals can be had on Amazon for like $20.