Each repository in the system has a set of maintainers. These are
tracked in files all aptly named MAINTAINERS. One of these will
always be present in the root of a repository. Many directories will
have their own MAINTAINERS files.
Each file lists a number of individuals (via their email address) who are familiar with and can provide code review for the contents of that directory. We strive to always have at least two individuals in a given file. Anything with just one is either too fine grained to deserve its own, or is in need of someone else to learn enough about the code to feel comfortable approving changes to it or answering questions about it.
MAINTAINERS are not OWNERS, and in particular code review +2 is not gated on membership in the appropriate file.
Rather, these files are about being able to find an individual who knows about and is responsible for code in the system. I.e. someone who can answer questions about the role of the code in the system, or who can review changes to the code in question (or at least provide more appropriate reviewers).
INTK-108 tracks the work to stand up more infra tooling around these, such as suggesting reviewers automatically in Gerrit.
We use the Gerrit find-owners plugin file format for our MAINTAINERS files.