I assume the videos available on iTunes are encoded in an H.264 profile that can be decoded entirely on the GPU, so the CPU would indeed be mostly idling during playback.
Yep, you'll also get H.264 served on Safari, while Chrome will get VP9 which isn't hardware decoded and will cause the fans to whirr.
There's a h264ify chrome extension that forces YouTube to serve H.264 to Chrome on OS X so it can be hardware decoded. It significantly reduces battery drain.
You get accelerated H264 in Chrome which you paid for (through Apple), but YouTube prefer to serve vp9 if you can decode it, to save their own licensing fees. MS Edge only reports being able to decode vp9 if you have hardware acceleration for it, for battery life.
Not so much licensing fees as bandwidth - VP9 as a new generation codec (H.265 being a rough equivalent) uses less of Googles bandwidth which probably saves Google a lot of money for a "minor" cost of users power draw :)
Most Apple devices don't have VP9 capable hardware decoders. Even those that could do it (new Intels) do not have such capability enabled or exposed in OS X.
There's a h264ify chrome extension that forces YouTube to serve H.264 to Chrome on OS X so it can be hardware decoded. It significantly reduces battery drain.