For two-pass encoding, the video is first scanned to assess complexity, so a second encode pass can work out which frames require a higher bitrate to get the sa

For two-pass encoding, the video is first scanned to assess complexity, so a second encode pass can work out which frames require a higher bitrate to get the same quality. However this is not used that often, in favour of a quantiser approach, which uses a number to indicate target quality. The encoder then changes the bitrate depending on the frames complexity and the target quality/quantiser. The quantiser approach is much more common because it's easier to specify an output quality that is independent of resolution. As for the actual encoding, nearly all the computation is very specific to the output resolution/quality. Luckily encoding multiple videos can be spread over many machines, and encoders can take advantage of multiple cores.