Where to see kernel startup messages after boot?
dmesg did not show me the entire boot log but only the last few 100 lines of it, which was not enough.

What works for me (on macOS 10.12.6) to see all boot messages is this command:

sudo log show --predicate "processID == 0" --start $(date "+%Y-%m-%d") --debug

It shows all of today's kernel msgs, including the boot process.

If you only want the msgs since reboot, enter the date and time manually, like this, in 24h format:

sudo log show --predicate "processID == 0" --start "2018-10-28 13:19:00" --debug

(The "sudo" part is currently not necessary, but it doesn't harm, either.)