Please refer to Gst-nvvideo4linux2 — DeepStream 6.1.1 Release documentation
nvv4l2decoder is HW accelerated decoder, so the buffer it uses is special.
colorFormat 2 means NVBUF_COLOR_FORMAT_YUV420 format, it is not ordinary YUV420 format, but the Nvidia HW adapted format.
For your example:
plana 0 size 8912896 = 4096 x 2176 because the width and height for HW should be the multiple of 16.
plana 1 and 2 size 2228224 = 2048 x 1088 also obey the rules of multiple of 16.
For CUDA gpu copy, please refer to How to use NvBufSurfaceCopy to copy surface from CUDA device to CPU accessable memory