How does flatpak provide access to Vulkan / GPU

I am running the flatpak version of Steam, and it happens that my system has two discrete GPUs, a nvidia one and a lowend AMD one. Normally Steam opens proton games and it uses the nvidia card for running Vulkan, but sometimes, specially after some upgrade, it uses the AMD one. I have tried to understand which GPU is passed to the flatpak container, or if both, how it decides which ones to use, but apparently there is no Vulkan ICD loader in the flatpak container. How does flatpak provides access to the GPUs of the host system? How can I enforce to use one or another?