I wondered how he was doing this without some sort of mold but after watching the first 5 mins of sandcastle building it clicked what he was doing. You’re right that was very mesmerizing and relaxing to watch. Thanks for sharing,
I watched far too many of these videos over lockdown
In case you haven't seen this one it has a breakdown of most of the how to here including making the scrapey bits and a cool diy potters wheel
They take way longer to cure than YouTube vids will try to convince you probably a lot to do with climate to be fair to the content creators but YouTube making things appear faster/easier and often skipping steps for ease of viewing is another thing I seen too much of over lockdown!
Apparently letting it set and then curing it under water for a few months is the best way to protect from winter damage but I'm yet to be able to prove/disprove this
Even wall thickness is important and I believe these scrapey things hold the key!