To request a review you push the changes into a special branch namespace in the reviewer's repository. IntelliJ uses / as a way to organize branches into folders, so it's easy to see what review requests come from who. You can also inform them in other ways (CCing on a ticket, email, chat, etc).

We use a kernel model in which there is no shared repository, so it's impossible for branches to be merged without being reviewed because it's the reviewer who merges them when they're satisfied.

How do you push changes to someone's repo? I think the kernel guys use email right? Sounds a bit painful.

git push. We use Gitolite which is a git server setup that can be made to enforce access control rules based on branch names.

https://gitolite.com/gitolite/index.html