Yes, these are hard-wired logic and just a handful of bytes for two to four registers (at least input and accumulator registers, everything in BCD). Only more advanced calculators use floating point.
I've been interested in how 4-function calculators and cheap clocks work for a while.
Most of the implementational details would appear to be buried in files on engineering workstations at Chinese factories.
I found a calculator teardown at http://electronupdate.blogspot.com.au/2016/08/reverse-engine... a while ago. The author says it's very old but doesn't provide a ballpark (eg, 1990, 1995, 1998, 2003, etc). It does look quite hairy (and possibly not particularly compactly designed?).
I would guess there's a ridiculously simple ALU somewhere in there, but I wonder what else is involved in the specific context of a high-production-volume fixed-functionality design.
I hope to find something similar for a clock at some point.
