While this may not be strictly “sound design”, I don't know of a community which this is better suited for. I have already done a preliminary search on this and always get different takeaways, none of which help my analysis of my music to optimize for loudness on these platforms.
Some things I know
has some kind of compressor. Integrated loudness can vary between 0 and -5ish LUFS. Peak LUFS seem to be able to range from 0 to the RMS level, depending on the mix. By boosting the bass I can get the integrated level up, so I speculate that I should be clipping my drums to the level of my bass+average mid sounds, so that the SPL stays at some fixed value no matter what drum is playing. Overall SoundCloud loudness seems like a skill issue...
YouTube On the other hand seems quite chaotic. Before the compressor was introduced, about 6.5 years ago from the time of this post on 2023/10/1, YouTube made no changes to audio at upload time. So videos from this era would be mixed to -5 to -7 LUFS. Examples can be found in much music from the early days of YouTube. With the compressor, YouTube wants peaks at about -9, and integrated at about -14, but there is certainly some leeway for many mixes. It seems if the audio is mixed well for SoundCloud, it will be loud on YouTube, but perhaps there are some tricks to know, given the specific conditions of YouTube.
Regarding YouTube, however, there seem to be people who can pierce the invisible firmament of loudness on YouTube. It appears that livestreams are not subject to the same level of audio level regulation as standard async video. Also, there are odd examples of people getting to -5 on a regular basis, as if they know a trick for the audio.
So maybe there is even another level on top of the mathematical game theory, where people are perhaps exploiting YouTube's technology somehow to get better audio. Sam Hyde seems to be able to do this on a consistent basis with his music uploads, which suggests this is not a fluke technical error by YouTube.
Would anyone be kind enough to explain to me what they know on this topic?