Immediate mode UI toolkits are designed for pluggable backends, some even can discover the appropriate backend at runtime. If you're writing a game, you're expected to (and actually, you must) build your own integration.

ImGUI and Nuklear each have 20+ backends in their repos: <https://github.com/ocornut/imgui/tree/master/backends> <https://github.com/Immediate-Mode-UI/Nuklear/tree/master/dem...>