For the one I posted, I used an AI image generator + another AI image to video converter + ezgif (which nostr:npub1pefpytg7h9wdmza97evpta73eyf94rq5mq5cn6h992eknm4xcljqq2tv2p also suggested, it's dank)

I also used to have one that was just a bit of footage of water flowing in a stream from Wikimedia, cropped/reversed/compressed with ezgif

I used webp to make these small enough but later learned webp might have security issues, so the really hard art form of this is making an actual animated gif that isn't too heavy to load, like the bitbees bee. You would still need ezgif