Run without --source 'https://rubygems.org/'.
Ruby will use it by default.
The gem install command might not be right it installs for your user. In your project, try:
bundle install ffi
Also on macOS, with Catalina, the system ruby and its gems are locked. So avoid using that ruby.
Also you didn’t use the --user-install flag on the gem install bundler which leads me to think you might have used sudo.
I wrote I guide here on installing Ruby, Jekyll and Bundler on macOS Catalina, installing Jekyll and Bundler at the user level.
Lastly, can you can try a different project.