Taken literally, kernel maintainers are the people listed in the
MAINTAINERS file. There are mainly two ways to get listed there: one is to add a subsystem to the kernel, and become its maintainer, the other is to take over maintainership for an existing kernel component. There was a recent example of the latter which followed an episode which generated some buzz, the potential removal of the floppy driver; Denis Efremov became the new maintainer.
In general, becoming a maintainer is a result of becoming involved in the general curation of a given component. If you do a good job, gain a reputation as someone reliable and trustworthy, and the appropriate circumstances arise, you’ll eventually have the opportunity to become a maintainer. This applies for small components (e.g. the floppy driver mentioned previously: Denis had demonstrated his ability to take good care of it before offering to maintain it), and of course for larger components, all the way up the hierarchy.
This also tends to apply generally, in free software / open source projects, but with different terminology: typically, “committer” status.