macos – Mac OS: where is dmesg log located?

The output of dmesg command is the standard output.
Its function is to format kernal internal logging buffers on the
standard output.

You may find the same informations formatted and dispatched inside
many different system logs, for example:

/var/log/system.log
/var/log/asl

Another command: syslog lets us display these messages in real time
before they are dispatched by syslogd in their appropriate log files.
Try for example:

syslog -w