The feature is part of the Twitter ads platform, just like Facebook's custom audiences. A third party partner on behalf of an advertiser or the advertiser thems

The feature is part of the Twitter ads platform, just like Facebook's custom audiences. A third party partner on behalf of an advertiser or the advertiser themselves can upload the list of identifiers (here phone numbers) to match against (the audience). Note that an audience can only be used if it matched at least 500 users.