There are various ways to take advantage of the GPU -- the so-called GPU-accelerated terminals contain bespoke OpenGL programs ("shaders") to do rendering on the GPU. Here's an interesting way of doing it: https://tomscii.sig7.se/2020/11/How-Zutty-works

Hmm I think it's more the case that an application has to go out of its way to not render on the GPU. High level frameworks' basic rendering operations are, of course, always GPU accelerated. They'd be crazy not to be! It looks like Foot is rendering using CPU by talking directly to the compositor.