GNOME and Linux are under the GPL. So are phosh (Purism's shell), Chatty (their messaging app) and probably a lot of other components. So you can't make a closed fork.
It's usually quite easy, they're often on Github though some companies host them themselves. There are a few companies (looking at you Lenovo) who release partial/non-working sources but most vendors do comply with the GPL.
It often seems to take some persistence, but typically it is possible to get the kernel sources. The manufacturers can drag their feet for a while but ultimately they have to give it to you.
Compare that to the rest of the software on an Android device; you'll have no luck convincing Samsung to give you the source to any of the UI bits on their phones, for example.
Linux specifically does not use the GPLv3 for exactly this reason. rms et al. complained about "tivoization", which came from tivo shipping boxes that contained the code but were locked down. Linus didn't really care as much, or at least not enough to re-license. He didn't like the restriction, and new that such a provision would prevent exactly what MS is now doing: allowing more freedom for the users.
That's orthogonal though. WSL would be fine even with GPLv3. It runs on a hypervisor. WSL poses no restrictions on modifying the source of the kernel. You can fork, rebuild and run. You can't on a tivo device.
Not exactly. Microsoft is trying to expand functionality in certain places to allow things like CUDA acceleration, AF_PACKET support, and other important hardware-y things that WSL can't handle. You're right that you can run a different kernel, but Microsoft has modified theirs for WSL. For that reason, they have to release modifications.
