pg_internal.init is a file in the data directory which contains the relation cache. A file exists in the global subdirectory, and one in each database subdirectory and possibly in each non-default tablespace.
pg_internal.init file is removed at postmaster startup, and recreated by the first backend which is launched.
pg_internal.init is defined as
The file is managed by src/backend/utils/cache/relcache.c.
See also: src/include/utils/relcache.h.