So my pc keep randomly crashing and rebooting, and I can’t find why because it happened very randomly. So I wonder what log file, log stream, or other stuff should I check right after the reboot to get what cause the crash?
Run journalctl -b -1 to view the logs from the previous boot.
That definitely a good one to know thanks 
Now I have a good bunch of logs to read!
Just to be sure if any program or system failure happened it should be there?
Unless it was a real sudden one yeah.