Where are syslog logs stored?

The following works for me:

#include <syslog.h>

int main()
{
	openlog("myprog", 0, LOG_USER);
	syslog(LOG_INFO, "hello world\n");
	closelog();

	return 0;
}

journalctl shows

jan [...] myprog[12907]: hello world

strace shows it connecting to the /dev/log UNIX domain socket as expected:

socket(AF_UNIX, SOCK_DGRAM|SOCK_CLOEXEC, 0) = 3
connect(3, {sa_family=AF_UNIX, sun_path="/dev/log"}, 110) = 0
sendto(3, "<14>Jan [...] myprog: hell"..., 40, MSG_NOSIGNAL, NULL, 0) = 40
close(3)                                = 0