This process initiates with Linus Torvalds, wherein, he releases a new kernel and then opens a 2-week merge window. During this merge window, he pulls the code for the next release from subsystem maintainers. Subsystem maintainers send signed git pull requests to Linus either during the merge window or before
No, that’s just for development discussion and code versioning. Nothing the end-user needs to touch or see.
End users update their system with commands/through interfaces or they receive automatic updates, depending on their system.
We’re talking about patching the kernel though.