Airdrops are of many type some airdrops are real but some are not real because we have to check on YouTube or by referring friends those who are in vesting in such type of activity.
Airdrops can be given on basis of new coin launching so they make ico and airdrop before coun has to launch so they start the ico and also help people to join in airdrop and so they will get free airdrop after successful launch the coin.
After launching the coin they start distributing coins of people who had invested their money in such ico.after that the people who had taken part in airdrop should given a free token on the ethereum address.
So I believe that mostly airdrops are real but few of them are said to be fake.beacuse they use dublicate website and start collecting money and they runs away.