You are correct. They can however stop doing business with whoever is distributing their source, which makes getting new versions of the source harder.
This is what the dude selling „hardened“ versions of Linux is doing. Can’t remember the name and I don’t care to give him advertisement anyway, but he simply stops selling you new versions if you distribute old ones.
I like looking for issues that are tagged as „good first issue“.
There are sites that make this even easier.
Look for projects that you think you might want to support in a language you know and help them out in those issues.
I like the idea and used Manjaro for a few years, but its run by less competent people than Id like (or at least in comparison to other distros), so I stopped and moved to a different distro.
Yeah, that’s it.