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Make video with alpha channel with those spiders, for example in webm format, then set video as Fixed position and 0 for top, bottom, left, right and width 100% height 100% and they will be always on top.
On your example, they use the same logic but instead of video, they use image sequence. Here is example of video https://www.pond5.com/stock-footage/item/876585-spiders-crawl-and-down-loop-alpha but this requires to have video editor to make proper loop where spiders move to random places