> Hm, where do I find the file containing sendto()?
> By a quick search I only found src/lib/libc/net/send.c which calls
> sendto() itself.

There is none; it's one of the many syscalls that use the default
implementation on all architectures.  See src/lib/libc/sys/Makefile.inc
for the ASM= assignment (which includes sendto.S) and the snippet

${ASM}: ${ASMDEPS}
        printf '#include "SYS.h"\nRSYSCALL(${.PREFIX})\n' >${.TARGET}

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