They use legal action as a tool to make that happen; in many cases you never hear about it because the company quietly settles and releases source code.
What I asked was " in many GPL projects there are numerous copyright holders. Therefore, who is going to be the one to sue over the issues and optional seek damages"
You responded with "they".
That's not addressing my point, which is asking about which actor is taking action, why they are and not another actor, etc.
Like, is someone who contributed 3 lines of code 7 years ago going to be actively suing people?
Yes, they could if they care too. Folks have done this with (relatively) small contributions.
Most companies are willing to share their code as GPL requires when asked.
What they aren't willing to do is provide authorization keys etc to hardware etc as they don't feel GPLv2 requires that. Because most GPLv2 devs ALSO don't think GPLv2 requires that not a lot of litigation there.
My guess is that if SFC does get a smaller kernel contributor to assign copyright the kernel devs may try and remove their contribution.