Supposedly they'll turn it down if it's mastered louder than -14 LUFS. They won't turn it up if it's quieter.
replyI couldn't find a Youtube source for this but it's mentioned extensively online: https://audio.rswaver.com/blog/youtube-loudness-standards
They have a "stable volume" toggle, actually. I don't see ads, so I don't know whether it works for those.
replyThey do, but as usual with those, it wrecks all the decently mixed videos by making everything the same volume.
replyThough as those are rare as hen’s teeth, perhaps you might as well.