A team at Universal Containers works on a big project and uses Yarn to deal...

If the dependency was added as a dev dependency (yarn add --dev), it goes into devDependencies.

When NODE_ENV=production, Yarn (and npm) typically skip devDependencies on install.

Thus the package doesn’t get installed for other team members in a production environment.

YARN_ENV is not a standard variable, --save is npm syntax (and Yarn saves by default), and --add is not how Yarn works (the command itself is yarn add).