Manual:$wgInterwikiCache

Details
 consists of the following four settings:


 * $wgInterwikiCache: This setting specifies path to constant database file. This  database is generated by  from maintenance and has such key formats:
 * dbname:key - a simple key (e.g. enwiki:meta)
 * _sitename:key - site-scope key (e.g. wiktionary:meta)
 * __global:key - global-scope key (e.g. __global:meta)
 * __sites:dbname - site mapping (e.g. __sites:enwiki)
 * Sites mapping just specifies site name, other keys provide "local url" data layout.


 * $wgInterwikiScopes : This setting specifies the number of domains to check for messages:
 * - Just wiki(db)-level
 * - wiki and global levels
 * - site levels


 * $wgInterwikiFallbackSite : This setting is used if unable to resolve from the cache.

This appears to use a constant database file (created by one of the maintenance scripts) to provide a list of valid interwiki prefixes and corresponding URLs.

Since  it is possible to get the list of cached interwiki prefixes, used in the  and  extension.