Here's a link to my complete article:
Below is a summary of the article.
A "system" Ruby comes pre-installed on macOS to support scripting and casual utility programming. MacOS Big Sur and Catalina include an older Ruby 2.6.3 (Ruby 3.0.1 is the newest version). To check if Ruby is installed and see its version, enter `ruby -v` in the terminal application. For details, see the article Is Ruby Installed on my Mac? Most developers use a software version manager to install a newer Ruby version for developing software on a Mac.