video – What does FFMPEG preset use under the hood?

FFMPEG has presets ultrafast, superfast, veryfast, faster, fast, medium, slow, slower, veryslow, placebo. I downloaded the source code. Did a grep -r "veryfast" .. Found ./libavcodec/qsvenc.h and in ./libavcodec/qsvenc.c, I found if (avctx->compression_level > MFX_TARGETUSAGE_BEST_SPEED). So is that it? Is it only used to decide the compression level?

Reason for asking: I wanted to use crf, maxrate and bufsize to limit the bitrate, but wondered if it could be used while using preset, or whether it’d override anything, if preset used any settings that influenced the bitrate. I haven’t (yet) seen examples where constrained encoding (VBV / maximum bit rate) uses preset. Wonder why.

In any case, it would be nice to know what preset uses under-the-hood. I see some settings here, but it’s unclear.