How does Google deal with external dependencies? E.g. the Linux kernel. Do they have a copy of Linux in the monorepo for custom patches? Is there a submodule-li

How does Google deal with external dependencies? E.g. the Linux kernel. Do they have a copy of Linux in the monorepo for custom patches? Is there a submodule-like reference to another repository? Is there a tarball in the monorepo, a set of patches, and a script to generate the Custom-Google-Linux? What happens when a new (external) kernel version is integrated?