Manual:$wgInterwikiCache

Details
1>Special:MyLanguage/Interwiki cache|CDB interwiki caching consists of the following four settings:

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.

This setting specifies the number of domains to check for messages:

Just wiki(db)-level

wiki and global levels

site levels

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 siteinfo>Special:MyLanguage/API:Siteinfo|API and  extension.