It's a bunch of small things: command line flags, whether a command line tool is even present at all, compiler built-ins / differences in behavior, headers possibly being in different places, compiler support for various language standards and more.
Even to this day, it's not uncommon to find libraries that won't compile with one of GCC, clang, etc. or even the same compiler but Linux vs MacOS.
It was even worse in ye olde times before package managers, I'm assuming.
EDIT: I forgot to mention that System V and BSD are two of the major families.
Both influenced Unix-like OSes far and wide, such as SysV style init scripts in certain Linux distros, MacOS being derived partly from BSD, Solaris being a continuation of SysV IIRC, and more.
There was a rough standardization in where certain things could be found, command line flags, etc.
Even to this day, it's not uncommon to find libraries that won't compile with one of GCC, clang, etc. or even the same compiler but Linux vs MacOS.
It was even worse in ye olde times before package managers, I'm assuming.
EDIT: I forgot to mention that System V and BSD are two of the major families.
Both influenced Unix-like OSes far and wide, such as SysV style init scripts in certain Linux distros, MacOS being derived partly from BSD, Solaris being a continuation of SysV IIRC, and more.
There was a rough standardization in where certain things could be found, command line flags, etc.