Unlike Windows based machines, MacBooks always use the AMD GPU when an external monitor is plugged in. I forget the exact reasoning, but it is the way Apple designed these machines.
In 2016 when they were redesigned I don't believe the intel IGPU could drive 4k @ 60hz, and Apple also wanted compatibility with the LG 5K display they partnered with. The best solution was to just force all external displays to be routed thru the AMD GPU.
I've seen articles about the low-power mode being rolled out to macOS Monterey, even for intel Macs. Hopefully that can reduce power consumption. I've also decided to do a thermal pad mod for the VRMs+inductors to reduce throttling, avoid higher top-side temperatures & lower fan noise.
In 2016 when they were redesigned I don't believe the intel IGPU could drive 4k @ 60hz, and Apple also wanted compatibility with the LG 5K display they partnered with. The best solution was to just force all external displays to be routed thru the AMD GPU.