In order to make it easier to the users of the project, many people include a link to the version control system of the project in the meta-data of the project. In most cases they use the home_page field. In other cases the link is embedded in the description field in the JSON file.
Our database indicates that about 32% of the packages have no VCS, however we have to take in account that our process only checks the home_page field (not the description) and until June 12, 2020 the system only recognized GitHub URLs. Since then it also recognized GitLab and BitBucket URLs.
In order to make it easier for every to be able to contribute to their favorite project we would like to encourage more project owners to include the link to their public VCS in their meta-data. Ideally there would be a specialized field for this (instead of re-using the home_page field), but for now the home_page will do it.
If there really is no VCS then finding and adding the link to it is probably not an easy task. One thing that can be done is to see if the same author has multiple packages, maybe some of the other packages have VCS links. Then the VCS of this package might be in a similar place. In addition on could just manually search the major Git hosting services.