Can you please explain what you mean by exchange BTC in fiat money? kinda confused me and i never heard this term before either :/
Fiat is a general term meaning "your country's money".
Most of them pay per post , let's say a signature compaigns pays 0.001 per post weekly , if you do 100 posts you will end up with 0.1 BTC / per week which is pretty decent since it's free and not too much efforts . Just make sure to not spam otherwise you will get banned
Just one correction here, getting paid in a signature campaign isn't "free", you earn the payment with your time and knowledge. And remember that posts that don't contribute to the conversation will quickly get you banned from the signature campaign or even this Forum - there's little tolerance for spamming for money.
With that said, signature campaigns are a great way to earn a little bitcoin on the side.