This directory exists to hold a variety of projects and libraries that we might publish as NPM packages. Typically used also elsewhere in Calypso and build on npm start.
For projects that can produce independent, binary-like outputs deployed elsewhere, see /apps.
If you want to add a new project or package into this directory, then add a new directory and follow monorepo -documentation.
Packages are built on Calypso's npm install so you don't need to build them manually, unless you are working directly on them.
If you must manually build a single package, run:
npx lerna run prepare --scope="@automattic/package-name"