AFAIK the changing thumbnails are not made by YouTube. The creator uploads a number of options for thumbnails and video titles, then YouTube randomly? shows those options to users and provides statistics to the creator so they can apply the best performing (in terms of click through and watch time) option to improve video performance. It’s all of course linked to how the algorithm works, so is indirectly influenced by YouTube.

Source: a video I watched a while ago but I don’t even remember who made the video, so take what I say with a grain of salt.