They all have files that end up as /usr/include/sys/syscall.h and define well-known numerical constants for each (documented!) system call.
https://github.com/openbsd/src/blob/master/sys/sys/syscall.h
https://github.com/lattera/freebsd/blob/master/sys/sys/sysca...
http://cvsweb.netbsd.org/bsdweb.cgi/src/sys/sys/syscall.h?re...
Oh, look! They even share the exact same numerical values for the old school system calls. #1 is exit(), #2 is fork(), so on and so forth.
Gosh, on my linux laptop, here in /usr/include/asm/unistd_64.h is pretty much the same numbers, but objectively hidden a little bit more than in the direct Unix descendants. I interpret this as "less documented than in Unixes".
Wow, even Minix source code has the same numbers: https://minixnitc.github.io/posix.html
I wonder why that is? Maybe it's because the system call numbers are in fact documented, part of the POSIX API, and descend from Bell Labs Unix source code. Here's V7's list of system call numbers as proof:
https://minnie.tuhs.org/cgi-bin/utree.pl?file=V7/usr/include...