At runtime, GSettings looks for schemas in the glib-2.0/schemas subdirectories of all directories specified in the XDG_DATA_DIRS environment variable. The usual location to install schema files is /usr/share/glib-2.0/schemas.
In addition to schema files, glib-compile-schemas reads 'vendor override' files, which are key files that can override default values for keys in the schemas. The group names in the key files are the schema id, and the values are written in serialized GVariant form. Vendor override files must have the filename extension .gschema.override.
By convention, vendor override files begin with nn_ where nn is a number from 00 to 99. Higher numbered files have higher priority (eg: if the same override is made in a file numbered 10 and then again in a file numbered 20, the override from 20 will take precedence).