Amethyst uses rendy for the rendering interface, which is built on gfx-hal (a very very low level rendering API), which in turn can use vulkan, metal (basically Mac’s weird version of vulkan lacking some features), DX12, and with an OGL fallback. So yes, it can be built and run natively on mac’s, though because of the way the mac ecosystem is broken to hell that has to be done ‘on’ a mac, which none of us have. ^.^;
Well the RPi4b supports modern vulkan, previous ones use OGL2. ^.^
Yep, it uses metal.