Perhaps 10 years ago, Roy Underhill demonstrated how those spikey recesses were
made.  He clamped on a metal plate with a cutout shaped like that in it, then he
had a tool with cutters at the two ends of what looked like a flexible tuning
fork and it was spun with a bow drill and raced all around within the metal
plate and cut the recess.  It is called a Parser Drill:

https://makezine.com/2015/08/05/old-tech-drills-any-shaped-hole/

Mesmerizing to watch

Ed Minch