I don't know how they choose the lectures that they put online, but I can tell you that the effort needed to do so for a lecture is not at all trivial.
From having researched things a bit myself, you need (at least) an in-class cameraperson to film the entire lecture and then you need post-processing. You need TV-grade equipment, for both audio and video. You need to ensure that the room is noise-free and has good lighting. The instructor ideally also needs to be aware of the recording (at the very least, they should at least not be standing between the camera and the board).
From having researched things a bit myself, you need (at least) an in-class cameraperson to film the entire lecture and then you need post-processing. You need TV-grade equipment, for both audio and video. You need to ensure that the room is noise-free and has good lighting. The instructor ideally also needs to be aware of the recording (at the very least, they should at least not be standing between the camera and the board).