What are these half moon shapes on the bottom of my original AWU2?

I noticed that when cleaning my AWU2 that there are these half moon shaped things that aren’t removable on the bottom of my watch. I’ve never submerged in water or liquid. I hadn’t noticed this before. This is the version of the AWU2 that still has the blood oxygen reading. It’s hard to tell but the large and small ones are symmetrical and the edges are smooth