Source code of the standard distribution is managed by a GIT server hosted by GitHub and source code for most third parties external modules are hosted on FramaGit(but any developer or company can use its own Git repository).
You can find anonymous or developers Urls to Git sources and code documentation on the page :
For Dolibarr ERP/CRM :
URL for sources: https://github.com/Dolibarr/dolibarr
URL for Doxygen documentation of sources: https://doxygen.dolibarr.org
For external third-parties modules:
Some statistics about Dolibarr standard sources are available here : https://www.openhub.net/p/dolibarr/analyses/latest