> Didn't Apple sell SDKs to developers? They could've just gotten their contact info
> from there. Plus there were things like software associations, like the ones that pop
> up in shareware programs.
Not initially. The II/II Plus/IIe basically came with a copy of DOS 3.3 (not to be confused with MS-DOS 3.3) and a bunch of manuals documenting the hardware, so people could and did make and sell software without interacting with Apple about it.
Later on there was a formal licensing program for ProDOS and GS/OS, but as late as 1989 companies were still shipping Apple II software on DOS 3.3 disks.