@CommodoreSpence @mos_8502 A former collegue of mine uesed to work for #Ariolasoft here in germany back in the 1980s and he told me, they were often using custom made interfaces to connect the C64 to a PC and cross-compile or cross-assemble the games. The binary blob would then be written directly to the C64 memory via that custom interface.
They also used to write custom character sets, sprite data and such stuff into the target machine (C64) in the same way.