nsswitch.conf, the Name Service Switch configuration file, is used by glibc and certain other applications. Most notably, it controls the user, group and host lookup processes.
RHEL 9, based on Fedora 34, has glibc 2.34 that supports automatically reloading nsswitch.conf if it is changed.
Since Fedora 36, nsswitch.conf is managed by authselect instead of being directly under glibc.