can you give us a real example of url link or referring site? how can you tell the airdrop channels is using referral system?
if they do, then most likely it's a closed referral program with pre-selected participants
or simply they get fixed payment (from the project) for spreading news and referring people to sign up
the project can tell its referrer by the traffic's origin (HTTP referer) or reading the cookie set by the referring website
if it's an open referral program, then usually each referrer gets a special referral link to use