Why YouTube processes videos

All digital videos have some form of encoding. When you upload your company's videos to YouTube, the site will re-encode the files to conform to its own video standards. This helps Google serve the videos to viewers more quickly and provide a product compatible across all types of computers. For the sake of ease of use, YouTube supports a wide range of codecs during upload, including Windows Media Video files and Apple's .MOV QuickTime files. Rather than play the uploaded files – which may have very large files sizes and not be compatible with all devices – directly back to viewers, YouTube re-encodes the files after uploading, further reducing file size and aligning them to a unified standard. YouTube also creates multiple versions of each video to allow playback at different resolutions.

source: https://yourbusiness.azcentral.com/mean-video-being-processed-youtube-23680.html