At work, DC power systems for the telecommunication market. Deceptively complex, think UPSes if they were made of Lego and scaled from kilowatts to hundreds of kilowatts, with installed bases stretching across decades.
At home, ghidra-delinker-extension [1], a Ghidra extension for exporting relocatable object files (or in other words, stripping programs for parts like Doctor Frankenstein). Absurdly complex, got me a poster session at SURE workshop at ACM CCS last year, as a hobbyist.