they do get warm. some programs such as Apple Compressor seem to cap GPU usage to 50% even when cold. but e.g. ffmpeg spikes up temp

It's the hardware, Mac is/was also the best Windows machine and Linux machine so it's not necessarily the OS.

Generally speaking Apple focuses on providing a small product line with a lot more thought going into the design of each device, both internally and externally. With the iPhone and its ecosystem of profit streams they enjoy a lot more freedom to do this than other companies, having the iPhone absorb most of their CPU R&D cost instead of paying into Intel and AMD profit margins alone probably makes a huge impact on their BOM.