For a distributed company with developers from all over the globe the "local" here doesn't really make much sense. But from my experience with NPM, you download packages on your developer machine once you set up a project, and then only when something really messes up node_modules, which happens once in three months, on average.

You do re-download packages for every build in CI pipeline as you build a docker image from scratch though, and that's when NPM mirror is usually set up.