Kernel Sources?

[Doom Slayer]

How come there are ROMS which are CM based on the device which doesn't have sources?
I am talking about helium.
I believe that some devs can get their hands on sources, but they won't post,since there are some licensing issues, some things that cannot be shared.

Maybe there is an option to get a developer status?
Any advice on that one?

 

[Doom Slayer]

I also tried to contact Xiaomi, but they said no, without any explanation.
I used to develop for Sony Z Ultra, now I switch to Mi Max and want to get my hands on kernel.