More than likely youtube.
Unless the video receives traffic and views, youtube will encode the video in an AVC codec (which is a pretty bad codec) so you'll see pixelation regardless of what bitrate you record the 1080p video in. The better codec is VP9 , which produces a near lossless quality but requires one of two things.
1) Traffic and views (as stated above)
2) 1440p or above recorded resolution (this forces youtube to encode the video in the VP9 codec)
You can double check what codec Youtube has assigned your video, by viewing your video on youtube and
Right click on your video
> Click Stats for Nerds
> Check Codec AVC (standard codec) / VP9 (good codec)