It's funny, I just registered on this forum to get tips on the same topic. I actually know a decent bit about editing, encoding, and video streaming services (I work in IT at an Art School, and my father edited professionally for 20 years), but I know next to nothing when it comes to how YouTube encoding itself works. There also doesn't seem to be a lot of technical data/information about YouTube's back end encoding system either other than their basic suggestions (Which are rubbish IMO for high bitrate videos that motovloggers tend to put out). So far, here is what I have discovered, and it seems very contradictory to what a lot of people say ("Use the highest bitrate you can, and let YouTube down convert it!!"), but makes sense if you look at it from the sense that YouTube is really providing a free service to content uploaders (More on this in a sec if you're thinking about the ad revenue they get):
1. Higher Bitrate does not always equal better quality on YouTube's end.
What I have personally seen is when I upload VBS 2x pass at 45Mbps min/60Mbps max (FYI, these are the max bitrates for a GoPro 4 Hero Black with Protune turned on in case anyone is Googling for that right now, and stumbles upon this) with max render quality on 6-8min vids (Generally 1.5-1.8GB file size), the quality tends to deteriorate quite a bit from something lower like 15-20Mbps. While the quality of the local video file may look quite a bit better on higher bitrate video, once YouTube encodes it, the higher bitrate file seems to blur/chunk the video stream in the faster movement/higher data stream scenes compared to the lower bitrate video. I think this may be because YouTube's upload/encoding algorithm has a target file size that it tries to hit, and automatically encodes the video with settings that it estimates will hit that file size as well as the least server intensive encoding settings.
What does this mean? It means it's easier for a smaller video file to stay close to the same bitrate/settings that you encoded originally the first time around. Compare that 500-600MB file to the higher bitrate 1.5GB-1.8GB file that YouTube will have to squish who knows what way to hit the target size. This is all speculation as I have no idea if YouTube actually has a target file size it tries to encode to, but it sure seems like it based off the dozen or so upload tests I've done at various bitrates. Also, don't forget, YouTube offers the upload service for free, and has to pay to store the content somewhere. Sure, they make ad revenue, but probably not much from people like us motovloggers who have huge files, and a small subscriber base.
2. I'm really curious to know if YouTubers with higher subscriber counts/high view time get a higher encoding setting(s)/higher bitrate.
I've noticed most of the YouTubers (Motovloggers specifically) who have over 30k+ subscribers tend to have way better quality videos than some of the other stuff I've seen out there. I used to think it was just due to experience, and the know how. Since I started uploading though, I've talked first hand with a number of high sub and low sub motovloggers, and can't make heads or tails what is actually going on once people start uploading to YouTube.
Most of the high sub motovloggers I've talked to use bitrates anywhere in the 12-20Mbps min range, VBS 2x pass, max render quality, etc, but their video quality is crystal clear on YouTube. While a few of the other low sub motovloggers I've talked to upload a video with either higher or similar encoding settings, the quality is inconsistent as hell. I notice on my videos the pavement/street tends to pixelate a lot, and is generally potato quality. I have some examples of this on my channel. My first vid I posted is low bitrate "But Better Quality" (20Mbps min/20Mbps max, VBS 2x pass, max settings), and my latest vid is high bitrate "Potato Quality"(45Mbps min/60Mbps, VBS 2x pass, max settings).
Can anyone fill my knowledge gaps with answers? I would love to get more insight into this whole YouTube encoding mess.