We could not find standard test protocols in the scientific papers about sports sensors:

90.3 Summary of the testing protocols of different studies

We could not find standard reference devices:

90.8 Scientific accuracy study papers with research grade ECG as reference

90.09 Scientific accuracy study papers with sports chest strap as a reference

And methods:

90.10 Scientific papers which determine the signal quality of HRMs

We found strong fluctuations in the accuracy of the different Apple watch models, which does not correlate with the model type. A newer Apple watch model does not correlate with higher accuracy:

19.19 Table of Accuracy Values for Polar OH1, Apple Watches, Polar H7 & Polar H10

We do not know of any scientific papers which confirm results from other papers about accuracy of sports heart rate monitors in a precise way.