This will depend solely on what project you are referring to. Different projects are hosted on different platforms, so where you file bugs and feature requests will depend.
Usually a project is hosted in only one platform (hosting in two doesn't make much sense and is a maintenance nightmare, unless one of them is just a mirror). For example The LaTeX Project now hosts the LaTeX kernel (2ε and 3) only at GitHub: https://github.com/latex3, though we also accept bug reports to the LaTeX Team and LaTeX-L mailing lists (and since several members of the team are here, bugs that show up as questions in this site are also addressed, although this site should not be used to report bugs).
Another example is TeXLive, which is hosted in a TUG-provided SVN server (svn://tug.org/texlive), but is mirrored to GitHub. However nether accepts bug reports and feature requests. This should be sent by e-mail to the tex-k mailing list.
If you're looking to report a bug in a package, the best place to look for that is the package documentation and/or its CTAN page. Usually package authors include contact information and directions to bug reports in the package documentation, so that's a clear shot. Else, if the author has supplied this information when the package was uploaded to CTAN, the pagkage's page has links to its home page, bug tracker, and repository, or at least the author name so that you can do a little stalking in the internet to find out contact information (if it comes to that ;-). Also, one place to find out who the maintainer is and where the package is hosted is this list.
There is also, of course, the case when the package is unmaintained. In this case you are out of luck. A good example is tabu, which is “for donation” to someone who wants to pick up its maintenance.
TeXStudio, specifically, has moved from Source Forge to GitHub in early 2018. The project home page (https://texstudio.org) has this News entry:
2018-02-03 Active development has been moved to github.com. We are happy to accept pull-requests and any other help.
Their site also says that old versions of the program can be still found in Source Forge, but now all issues and pull requests should go to GitHub.