I don't know about other distros, but Debian makes it extremely easy to download both the binary package and the source package. For instance, on the page for the jq package [1], you can download the source using the links down the right-hand side, which includes the full test suite. The key, in my view, is that Debian has a nice way to associate both the final output artefact and the source (both the original source and their patches) with a specific version.
[1] https://packages.debian.org/bookworm/jq