Cycles uses 100% of all GPUs and CPUs you have if you set it up to do so and if you render something complex enough to take some time. It is possible to set it up to use CPU rendering only and that is great if you have a powerful multicore CPU and huge scenes that do not fit into VRAM of your GPU, but you certainly do not have to use only CPU and most people use GPUs for rendering with Cycles.
Hit F4 followed by p and go to System tab to set up Cycles Render Devices:
You need to have a supported GPU(s) for Cycles to use it with up to date drivers installed. Nvidia GPUs are generally better for this.
If you render really simple fast small frames, the bottleneck might not be actual rendering on your render devices but preparing, loading and processing the scene and/or compositing and saving the renders so there are cases where GPU might not get the chance to work hard.
As for EEVEE it's a GPU renderer, so it will always use GPU for rendering.