As tech companies grow, they often find the need to develop multiple projects or applications in order to better address their challenges. Along with its main app, Deezer has also released Deezer for Creators, Audiobooks by Deezer, Android-TV apps, and (soon) a public SDK. Many other businesses have expanded as well - you can think of Uber with Uber Eats or BlaBlaCar with BlaBlaCar Daily for instance. These applications share a lot of common code, from authentication to playback engine, to Design System. Is there a way to factorise it? At Deezer, we chose to publish our code in libraries, which led us to reflect on: - the content of these libraries, - the extraction of features and components into a library, - the organisation of the libraries (e.g. single or multiple libraries, library modules), - the versioning and the handling of repositories (monorepo vs multiple repos), - the release of libraries and their integration into projects. Our return on experience will help you discover possible solutions, better understand the pitfalls to avoid (as we fell into some of them), and see how private libraries could benefit your own organisation.