Where is the Report

As per this article, Create Diagnostics Report will output to /var/tmp. It takes a few min to finish running, and will automatically open a Finder window to the file when it's done, the filename will be of the form "Wireless Diagnostics_SERIALNUMBER_DATETIME.tar.gz"

The other option, Enable Wifi Logging, will more verbosely log to /var/log/wifi.log. You can then view this with Console - in the sidebar select /var/log and then choose wifi.log in the next pane. Clicking the Now button in the upper left toolbar will scroll lock the log you see the latest output.

Alternatively you can use Terminal to monitor new entries, for exampe with tail

tail -f /var/log/wifi.log

Be sure to disable logging when you're done as the file will quickly grow in size

/var/log/ is the standard location for log files in macOS. In this case we are looking for /var/log/wifi.log Old versions are stored as wifi.log.0.bz2, wifi.log.1.bz2, etc

For viewing you can use the Console.app or Terminal.app with less or cat.

For reference, /tmp gets cleared at shutdown so its not a good place for log files. LOL