@hpb-htw V2 uses Lerna to manage each package, including building and publishing the distributions to NPM. See the link in the main README.
As part of the build process. Lerna calls the build step of each package. This step uses browserify.
If you switch to the V3 branch then you will find that The build step is using rollup.
So, be careful. Don't mix the versions.
By the way, the CONTRIBUTING document explains how to setup, make changes, test changes, and submit changes back to GIT. Please follow the document.