I'd bet the left conductor at the top of the left breaker is tinned copper.

I'd also guess the meter combo is rated at 320 AMPs continuous, 400 AMP maximum rating. The size of the SECs has to either be rated for 400 AMPs or have load calculation support for smaller SECs. Though not universal, most AHJs won't allow smaller SECs on a residence. Homeowners just keep adding stuff. The heated towel racks and toilet seats, hot tubs, welders, etc. etc just blow away the load calcs that justify smaller SECs.

If additional panels have main breakers installed they don't need to be rated for 200 AMPs. If they are main lug only they do need to be rated for 200 AMPs