Heart rate readings from your Apple Watch (and iPhone, where applicable) live inside the Apple Health export. Each reading has a precise timestamp, a value in BPM, and the source device. Athletes use these for HRV analysis, doctors review them for arrhythmia screening, and quantified-self folks correlate them with sleep and stress data.
How to open
iOS Health app shows them in the Heart Rate section. For analysis, extract from export.zip and load into a spreadsheet or analytics notebook.