https://www.mediawiki.org/w/index.php?title=Template:Recommended_Steps_to_Participate_for_Outreach_Programs_Mentors (used by the GSoC/Outreachy mentors' guides) currently says:
Help host new projects created during the program under the Wikimedia umbrella on [https://github.com/wikimedia GitHub] or [http://gerrit.wikimedia.org/ Gerrit].
It looks like there are a few resources on MediaWiki.org related to this topic:
https://www.mediawiki.org/wiki/Gerrit/New_repositories
https://www.mediawiki.org/wiki/Gerrit/New_repositories/Requests
https://www.mediawiki.org/wiki/Git/Creating_new_repositories (for administrators)
https://www.mediawiki.org/wiki/Manual:Developing_libraries
But, there isn't a resource that lists all the venues in one place and recommend when to use what. Like when to use:
- https://github.com/wikimedia
- https://github.com/wikidata
- https://github.com/toolforge
- https://github.com/wmde
- https://gerrit.wikimedia.org/
- https://phabricator.wikimedia.org/diffusion
(Might be more venues that I am not aware of Gitlab, SourceForge, etc)
This resource might be helpful not just for outreach program candidates or newcomers but probably for the broader Wikimedia technical community, especially for folks who are into developing tools, bots, etc.