Time-sensitive media, which is part of “higher education services“, should only be accessible on demand (see question number 2). The required adjustment is the accessibility adjustments offered by the platform (i.e. YouTube). In other words, there is no obligation to make it accessible if the platform does not offer a way to make it accessible.
On the public websites of the institution, it is mandatory to upload accessible videos (including subtitles). For example, the main website, faculty websites, videos about events taking place at the institution, instructional videos even if these videos are shown through the YouTube channel.