I have just noticed that I have two instances of Audacity 3.7.7.0 installed on my Mac:
Audacity app alone, at the root of the Applications folder;
But Audacity is also installed in a folder /Applications/Audacity/ In that same folder, along with Audacity, there are several sub-folders, i.e.: /help, /nyquist and /plug-ins.
Which of the two installs should I remove?
A standard installation of Audacity is #1 in your list. You should be able to delete the Audacity folder and its contents.
Are you sure the Audacity app inside the Audacity folder is version 3.7.7.0? Those sub-folders sound like they belong to a very old version of Audacity.
Yes, I confirm that the Audacity app inside the Audacity folder is version 3.7.7.0. It was updated over time. But I suspected indeed that it belonged originally to an old version. However, in order to be safe, I wanted to make sure I was right. Now, thanks to your answer, I will go ahead and delete it.
It doesn’t hurt to delete that copy. All settings of Audacity are stored in a separate directory - on a Mac this is ~/Library/Application Support/audacity. You’ll not lose any of your settings.