c:a libopus tells FFmpeg to encode the audio in Opus. c:v libvpx-vp9 tells FFmpeg to encode the video in VP9. Create high-performance end-to-end hardware-accelerated video. Granular control over encoding settings such as encoding preset, rate control and other video quality parameters. FFmpeg supports following functionality accelerated by video hardware on NVIDIA GPUs: Hardware-accelerated decoding of H.264, HEVC, VP9, VP8, MPEG2, MPEG4*, and AV1. Oposite from what most, atleast me, would expect is. qmax and qmin are the 'quality-ranges' in which you define to encode. for x264 it should be a quantizer see here, But with some others codecs it doesn't represent a quantization level but a quality range. When no bitrate is specified, vp8 inherits ffmpeg's default bitrate of 200 kbps.But with ffmpeg qmax may have different meaning as it's a codec dependent parameters.
it encodes a frame to maintain the specified quality and then compresses it further, if required, to keep it under the specified bitrate. VP8, by default, implements CRF as constrained quality i.e. b:v 0 is the correct way for CRF encoding in VP8.
