Related: The SymbOS operating system and graphical desktop for Amstrad CPC, MSX and some other Z80 machines was coded entirely manually - the first release was in 2006. https://www.symbos.org/

Of course for the Commodore 64 the GEOS desktop was already released as a commercial product in 1986. https://en.wikipedia.org/wiki/GEOS_(8-bit_operating_system)