How can I find the reason for my Mac's recent "restart"?
I use Terminal a lot, so my answer will include commands used in that application.  I know Console provides details, but I rarely see it as more than a GUI to access the logs.  Terminal can be found in the Utilities folder which is found in the Applications folder.

On newer macOS (macOS 10.12 and later) with the unified logging system, the processing of the text files doesn't show the whole story, so use the log show command below:

log show --predicate 'eventMessage contains "Previous shutdown cause"' --last 24h


Of course, expand the time if your restart happened more than 24 hours in the past.

After a while, you should see a message which contains a number indicating a reason.  To find the meaning of that number, you can use this site:
https://georgegarside.com/blog/macos/shutdown-causes/.  For example, my last shutdown cause was "-128" which, unfortunately, is "unknown".

On older systems, you should be able to find the text "Previous shutdown cause" in your system log.  You can find this by using this command (which you can copy/paste into terminal):

grep "Previous shutdown cause" /var/log/system.log