The source code of ERP5 is hosted on Gitlab. To use or download ERP5 either check for the latest (evaluation) version and sources on the release overview or get a production-ready instance through the developer getting started section.
ERP5 consists of a core which is extend by function applications, called business templates/bt5. You can follow the how to install business templates to extend your instance with any of the available business templates/applications.
As ERP5 has a large codebase, conventions and guidelines play and important role when contributing to ERP5. The following guidelines are relevant for making contributions to ERP5 and can also be found in the developer guidelines: