I do agree that earning cryptocurrencies for free (without investment) can be done, but it's really time consuming and depends on a lot of efforts and time available.
It's not worth it. Most airdrops are just a means of collecting your personal data and contact information to be sold for profit in email lists or similar. At most, you might gain a handful of cents or a buck or two. Bounty campaigns are hardly much better. The vast majority fail to ever make it to an exchange, and of the ones that do, the vast majority of them are similarly worth a few bucks at best. The chances of making serious money on airdrops or bounties is minuscule.
A much faster and more reliable way to earn cryptocurrency is to get a regular job, and either be paid in bitcoin or use your fiat earnings to buy bitcoin. You also have the added bonus of not being hated by all your friends and family for spamming their social media with ads for trash coins, and not being hated on the forum for spamming your signature with one liners.