The kernel UAPI's for DRM/KMS/V4L2 are maintained and regularly evolve. If they were not they would be deprecated as dead code and removed from the kernel. There is documentation in the kernel source and since this is a fundamental and core part of the Linux kernel, there is extensive prior-art in kernel code.
Kodi uses mesa for 2D capabilities GLES/GL and opening the EGL context that video/gui are rendered into. Kodi is largely a big fancy wrapper around libavcodec (FFMpeg) which for an RPi4/5 uses the uses the v4l2_request (stateless) UAPI for HEVC and on RPi4 (but not RPi5) the v4l2_m2m (stateful) UAPI for H264 support. FFMpeg also supports fallback to software decoding, e.g. how H264/VP9/AV1 etc. are handled on RPi5. If implementing changes you'd also need to consider VAAPI too, and in the near future NVDEC.
I'm not sure what thoughts you're having, but IMHO anyone who needs to ask where documentation is and how it's done will not be successful in coming up with meaningful changes themselves. Even working with Claude Opus and similar AI tools requires a baseline level of domain knowledge to generate prompts that achieve anything.