Deepstream with own model

Besides the supplied nvidia models and some exact examples of for Yolo, I can not seem to find how to take to take a generic ONNX model and get it running under DS6.0.

I noticed that for example Yolo has a custom library, that i think is processing the output of the Yolo model and converts it into deepstream specific format ? (Can anyone confirm this? )

Say i find a way to convert the latest detector from RoboFlow (RF-DETR) to ONNX, what would be the next steps to getting it to run under DS6.0 ?
custom-lib-path=libnvdsinfer_custom_impl_rfdetr.so (in the nvinfer config)

How would I figure out what this library needs to do ? Who made the one for the Yolo one, for example, and how did they find out what this is supposed to do ? Is there documentation for this ?

then I think there is a parse bbox libray… and the same questions as above.
parse-bbox-func-name=NvDsInferParseCustomRFDETR

Thanks,
Tom