Looks like he took a 30 circuit MLO panel and backfed a 100 amp breaker for a main. Since there are no main lugs present, I am guessing that's a GE panel. They used to make them with a pair of main lugs that would occupy the footprint of two full pole spaces. My guess would be that he pulled out the lug module and replaced it with a breaker (hey, at least there's a main)!
I sure hope that strap holding the main down isn't made of metal because it sure looks like it's crossing directly over the bus bar.
I also like how he brought the armored GEC all the way into the neutral bar instead of stripping the armor. Maybe he just didn't know how to strip it.
I am not quite sure that I understand what the sub panel is even there for. It seems that the main panel still has plenty of spaces, although getting wiring to them might be a bit of a challenge.
Now, what about what appears to be a 14/2 NM exiting the left side of the big panel? It looks to either be cut off or embedded in the masonry wall.
"Shares in the wire nut factory"....Now that's funny!