This by no means a expert explanation. But as I understand it peertube and owncast are video sharing sites that operate much like lemmy but for video. So people start platforms ( like a instance on lemmy ) and then other people can join said platform and watch the videos , however like lemmy you can watch videos across all platforms ( I assume much like lemmy as long as the platform admins allow ). Both peertube and owncast say they are Fediverse projects and as such it should be possible to access the videos on other fediverse enabled applications although not sure how well this in implemented.
I have to say as youtube alternatives I think they fit the bill best as it is free to watch , open source and quite an elegant solution , clearly early days but none the worse for that imo
Under the hood I believe it is a distributed load system ( a bit like the BBC’s Iplayer ) but that is beyond my pay grade.