Update: Motivation for the question
The motivation for the present question is to determine whether it is possible for a cyclist 1- to measure the heart rate without touching the crown, which would require holding the handlebar with just one hand for a few seconds, 2- to connect the Series 5 to a head unit—thus avoiding to look at the watch at all, and only glancing at the head unit, as well as 3- curiosity about how the Series 5 can measure the heart rate through electric sensors, given the understanding that there must be two electric sensors, quite far apart.
Cyclists and runners may want to keep track of their fitness level by confirming that their heart-rate remained below a certain threshold for a given activity. For that use case it's helpful to monitor the heart rate after a workout. But they may also want to ensure during a workout that they cross a heart rate threshold and remain there for some time.
A heart rate sensor that attaches through a chest band is a reliable solution, but it's far from being an unobtrusive one.
A good alternative, one that's much easier to tolerate, may be a wrist watch such as the Apple Watch Series 5.
Does the Apple Watch Series 5 have electric sensors instead of, or in addition to, optical heart-rate sensors?
The reason it matters is that electric sensors require the cyclist or the runner to stop and touch the crown with a finger from the alternate hand. (This produces an ECG, not just a heart rate, but the latter can obviously be derived from the former.) For continuous monitoring, IIUC, an optical sensor is still required.