It doesn't have to be complicated. Think MS Surface Book. Motherboard, CPU and small battery in the detachable screen. Plug into the base and you get more battery and dedicated GPU.
If I run games on the tablet I get a certain level of performance, if I run in laptop mode I get more.
This could readily be applied to a console. There would be enough hardware in the screen to run at a resolution and frame rate appropriate for the screen and power envelope. With access to a bigger GPU, more RAM? and fixed power supply in the base, you get a higher frame rate, resolution and perhaps improved effects. Games are developed for the full spec machine and scaled down for the handheld hardware.