YouTube mostly uses VP9 and H.264 / MPEG-4 AVC video formats, as well as dynamic adaptive broadcast protocol HTTP:
VP9 is an open and royalty-free video encoding format developed by Google. VP9 is the successor of VP8 and mainly competes with MPEG video with a high coding level (HEVC / H.265). Initially, VP9 was mainly used on the Google YouTube video platform. The emergence of the Alliance for Open Media and their support for the further development of the AV1 successor, in which Google is involved, has led to an increasing interest in the format.
Unlike HEVC, support for VP9 is common in web browsers (see HTML5 video § browser support). The combination of VP9 video and Opus sound in a WebM container provided by YouTube will be supported by approximately 4/5 of the browser market (from a mobile phone) as of June 2018. These two cuts among the big browsers are those set by Internet Explorer (as opposed to its successor Edge) and Safari (both desktop and mobile). Android supports VP9 with version 4.4 KitKat.
H.264 or MPEG-4 Part 10 Advanced Video Coding (MPEG-4 AVC) is a block-based compensation-based video compression standard. Since 2014, it is one of the most commonly used formats for recording, compressing and distributing video content. Supports resolutions up to 8192 × 4320, including UHD 8K.